Watching Ice Cream Sandwich, aka Android 4.0, get unveiled this week, I got chills. ICS is a stunning update — no, reboot — of the Android mobile operating system. So much was announced that it actually took me a day or two to digest it all. And now that I have, let me dive into this.

Judging by the presentation, this software update seems to be all about Google answering its critics, and the company did so with a stunning veracity. Fragmentation? ICS unites smartphones and tablets under one OS to rule them all. Clunky? Design changes sprinkled all over the place, across the whole system — from a new lockscreen and “magaziney” tiles in the People app, all the way down to a custom-created typeface (Roboto) that’s attractive and easy on the eyes. “Only a scientist” can use this? Oh puhlease. Ice Cream Sandwich took that notion and pantsed it in front of the world this week.

Indeed, it looks like Google really zeroed in on the user experience this time around, with an incredible number of flourishes and touches to make the operation smooth and — dare I say? — fun. There are new gestures implemented consistently across native apps and processes, including sideway swipes and flicks to close. Widgets are no longer buried in a hidden submenu, but get their own tab in the App Tray and resizing abilities. As for apps, organization gets a boost with homescreen folders, while a new favorites tray keeps your most-used applications close at hand.

Worried about data usage? (Who isn’t these days?) The new data usage setting features a chart (that lets users interact with it to drill down and pinpoint data-hogging applications), and customizable alerts and usage thresholds (to prevent the device from exceeding data caps). There are numerous changes that impact end users on a daily basis, from sexy things — such as fast access to the camera, tons of photo editing features, a new native panorama mode and live vid effects — to smaller things, like re-thinking text input to include drag-and-drop editing and enhanced speech-to-text.

Behind the forward-facing cosmetic and UI elements is also a robust back-end. Though Google says it does work with single core processors, Ice Cream Sandwich is actually optimized for processors with multiple cores. Given that this is the way the industry is heading, this could mean ICS won’t be hitting the obsolescence dump anytime soon.

Granted, eagle-eyed smartphone fans can see that it took some cues from WebOS, iOS and Windows Phone. But all tech companies are influenced by others to some degree — you didn’t really think Apple invented iOS 5‘s speech command or drop-down notifications, did you? — so let’s leave this one to the patent attorneys to chew over. Instead, as end users, our primary concern might be whether the company was able to take the good ideas — like cards, flicks, app folders, screen capture process, and tiles, etc — and integrate them elegantly. So far, it seems like Google did.

That’s not to say that it wasn’t innovating in its own right. What other tech company has used facial recognition in a mass consumer–oriented mobile platform (Face Unlock)? Or is jumping in to take Near Field Communication to the next level (via Android Beam)? Now, let’s talk about these two hallmark features for a moment: If nothing else, it was a gutsy move to push them into the spotlight like this. They each have potential to be defining features for this handset (the Galaxy Nexus) and for this version of software.

Unfortunately, there’s also plenty of doubt about how well they will work, and even if they do, how useful they will be.

Facial recognition technology is a tough thing to master — even law enforcement agencies and Homeland Security can’t say that their advanced systems are fool-proof. So what chance does a smartphone have? Could my friend, a real-life doppelgänger often mistaken for my twin sister, access my phone? Or could a photo fool the software? Well, at least for the latter, Google said yes… and um, then it said no. At this point, all we know is that the demo tanked at the press event, which may not bode well, but we’ll have to check it out for ourselves to be certain. (I’m also interested in confirming that Face Unlock activation is user-definable. I assume that it is, and that would be key: If users don’t want it, they’d probably like to know that it can be turned off, to ease any security worries.)

As for Android Beam, if you’ve ever beamed info via an old Palm Pilot or used today’s Bump app, then you should be pretty comfortable with the concept of close-proximity sharing. But instead of using infrared or an application that hooks to a server connecting to another user’s device, Beam uses NFC — which means even a cool utility like this could be seriously handicapped if other makers are slow to adopt this.

Having said that, though, who can’t appreciate the envelope being pushed like this? And even if these two features fall flat on their faces, there’s still plenty in Ice Cream Sandwich to savor. By all accounts, this looks like an extremely worthwhile update… well, at least for those who can get it.

And that’s probably the biggest issue for now: Who will be able to snag this update? And how well will it run on older devices?

ICS may have addressed some fragmentation concerns, but it’s not a magic wand that makes it all go “poof!” and disappear in one fell swoop. Google has said that (theoretically) all Android 2.3.x devices should work with this, and it will hopefully not be long before we find out for sure — at least at some point. Obviously, that will be after it debuts on the Galaxy Nexus. Word has it, the Droid Razr, Bionic and Nexus S will also be getting ICS, though at different times. (For me, I’m supremely interested in knowing how the Nexus S in particular will perform — of the three mentioned above, it’s the only one with a single core processor.) But the timetable is anyone’s guess. There are carrier hurdles, custom skins/UIs like HTC’s Sense, and other considerations that always make Android upgrade paths a huge guessing game. (If you want to know more, ZDNet compiled some of the vagaries around this issue here.) As for tablets — well, apart from emulators, we have yet to see how this really runs on them yet, so we’ll have to reserve judgment on that aspect — at least until the Asus Transformer 2 or Motorola Xoom come out.

Android 4.0 emulator on a tablet, courtesy of This Is My Next/Verge

The bottom line is this: ICS may not be perfect, but what we saw at the press event looks incredible. If the real-life experience measures up, then it looks like it will be the very best version of Android to date.

I remember being on the ground at the T-Mobile launch party announcing the very first G1, and thinking that the OS showed a lot of promise. Since then, I’ve been lucky enough to try various incarnations across different brands and carriers — including the first Motorola Droid, HTC Hero, EVO 4G, Inspire, Samsung Moment, Continuum, Nexus, Moto Xoom, etc… Seeing Ice Cream Sandwich now, I have to say — I’m a little verklempt. Kind of like witnessing a little one’s first tentative steps, then seeing them grow up and come of age.

It won’t be long now until we get to see for ourselves if this really has fully matured or not. I can’t wait.