When the news broke this morning about Apple finally selling its iPhone 4 unlocked in the U.S., I imagined a lot of T-Mobile iPhone hopefuls’ first thought was, “Huzzah! Finally!”, followed quickly by “Um, how do we get around that pesky SIM card issue?”

If you’re not aware, GSM handsets (like those on AT&T and T-Mobile) rely on standard-sized SIM cards, which identify the phone for use on the appropriate networks. The iPhone 4, however, has a smaller one called a Micro SIM card. The GSM Association confirms that they are electronically the same, but physically, there’s a small, yet important difference — the size of the plastic card that the chip is embedded on.

And that’s the rub. The physical size difference means you can’t just pop a standard-sized T-Mobile SIM card into an iPhone 4. The Apple device’s physical tray is too small.

Standard SIM cards are 15 x 25 mm; the Micro SIM is 12 x 15 mm.

People with jailbroken and unlocked iPhone 4 devices have had to deal with this issue since last year, so luckily plenty of solutions have already been worked out. And all of them deal with one main concept: Since the major difference between SIMs and Micro SIMs is the spacial area of the plastic surrounding the chip, trimming down a standard card down to the micro size should work fine without any loss of function. And it does — provided the user doesn’t slice through the chip.

But it’s important to note that the iPhone 4 doesn’t work on T-Mobile’s 3G bands. So even if all goes well, don’t blame the hack when the device only performs on EDGE. This is an inherent hardware limitation of this (and previous models).

If you’re okay with that, then read on for different ways to approach this chop job.

Micro SIM cutters

Some people get queasy at the thought of putting knives or blades to their SIMs. When product manufacturers got wise to this, they started making hand tools designed specifically for the job. The benefit of these is that there’s no measuring or precision skill required. And if you’ll need to do this job more than once, this makes easy work of it. Just follow the directions, clamp down and you’re done.

Here are a few sellers who offer Micro SIM cutters:

(Get yourself an adapter too. Some of the sites mentioned also sell a SIM adapter that will up-size your new Micro SIM, for use in phones with regular SIM card trays. These are only a few bucks, and are very handy to have around. I have one, and after a year of constantly popping it into a variety of phones, I haven’t had any issues yet.)

Stick-On Cutting Guide

If you have no need for a dedicated tool and want to cut it yourself, but need some guidance, you could also apply a sticker with cut lines to your T-Mobile SIM card. European seller SIMCut offers an adhesive guide for about $10 USD.

Freehand Slicing

If you’re exceedingly sure-handed, there’s always the do-it-yourself approach.

No, not like that! I mean, yes, this guy uses a cleaver to score cut lines, but the actually snipping is done with scissors.

Here’s another DIY guide from TouchUserGuide. It was created last year, to deal with the iPad 1’s Micro SIM card slot (which was the first), but the measurements and technique apply for the T-Mobile card as well. There are many other guides online as well, so don’t be shy about scouting around.

A Favor From a Nice T-Mobile Rep

I called T-Mobile today, to find out if the carrier can offer any help for would-be iPhone 4 users. Predictably, the customer service rep couldn’t confirm whether an official T-Mobile Micro SIM card would be available, and as of yet, there’s no official company policy regarding the alteration of their current SIM cards. HOWEVER, the lady offered some interesting advice.

She casually mentioned that many store reps are willing to help customers trim their SIMs. They aren’t bound to do this by any corporate doctrine, but many are simply used to lending a hand. This isn’t surprising, since Tmo has always been a friend to iPhone unlockers, and many have likely seen their fair share of Micro SIM trays since last year.

Of course, your mileage may vary — it depends on finding a good-hearted T-Mobile representative near you. But if you’re extremely skittish about this, it’s well worth a shot.

Have you ever cut down a SIM card? Let me know if there’s any other resource missing from this list. And weigh in, if you’ve got any advice of your own to share!