TUAW reports that T-Mobile has officially stopped allowing unlocked iPhone users on their network to use a prepaid Sidekick data plan.

Responses to questions from T-Mobile USA’s Twitter account state, “You will no longer be able to use an IPhone [sic] with the Sidekick data plan” and “The Sidekick Data plan is meant for a Sidekick device. You may want to look into changing your data plan if ur [sic] using an iPhone.”

Some MacRumors forum members on the prepaid Sidekick plan have reported that they have not been able to access T-Mobile’s EDGE data network in the past few days.

As the only other major GSM service provider in the U.S., iPhone users not wishing to move to or stay with AT&T turn to T-Mobile for service, despite its small yet still-expanding 3G network that transmits on a frequency incompatible with the iPhone 3G and iPhone 3GS. Up until now, the prepaid Sidekick data plan offered unlocked iPhone users unlimited internet/data and text messaging within the U.S. for $1 per day on top of any per-minute charges for calls. As a prepaid plan with no long-term contract attached, unlocked iPhone users had flexibility combined with reasonable plan costs.