Customers who contacted T-Mobile support received mixed answers. Representatives stated that service was down in their area or that they needed to upgrade to their normal data plan.
Yesterday, T-Mobile sent a tweet on their official Twitter account that they would no longer service iPhone customers using the prepaid Sidekick plan. So just like AT&T cutting off prepaid GoPhone service to iPhone customers, T-Mobile has now eliminated an affordable data option for unlocked iPhones. At this time, it appears that Android-specific data plans remain unaffected.
TUAW has contacted T-Mobile for an official statement on the matter but we have not heard anything back yet.
Thanks Gabe for the tip!