Sprint iPhone 4SAs we’ve all heard by now, Sprint went to great lengths to acquire the iPhone and, not surprisingly, the move has paid dividends. The nation’s third largest carrier had its best launch day sales ever, with consumers jumping on board to experience Sprint’s unlimited data goodness.

However, almost a month removed from the iPhone 4S launch, and customer complaints regarding slower-than-expected data speeds are growing louder. The Now Network is said to be working closely with Apple on the issue, but so far neither has a solution.

“We’re taking the reports of problems from a small number of customers seriously”, Fared Adib, vice president of product development for Sprint said in an interview with CNET. “There’s nothing of significance to report yet.”

The issue has become a popular topic among customers on Sprint’s support message board, garnering more than 1,200 responses and 235,000 page views. That’s a lot of traffic, although Sprint says that only a single-digit percentage of iPhone 4S customers have been affected.

More worryingly is the fact that Sprint hasn’t been able to replicate the issue, making it harder to address and fix the problem.

The iPhone is expected to account for twenty to forty percent of Sprint’s postpaid gross additions and upgrades during the company’s four-year, $15.5 billion contract with Apple, Sprint executives said, so a solution is of the utmost importance for the company. With the holiday season all but upon us, Sprint will want to quell the issue ASAP.

Have you experienced slower-than-expected speeds?