updated 09:43 am EDT, Thu June 28, 2012
Old data plans as cheap as $5
T-Mobile USA has confirmed, via a Twitter post, that people currently using the iPhone on its network will be able to retain any legacy data plans when it upgrades its cellular network. While T-Mobile isn't an official iPhone carrier in the US, and its 3G/4G network isn't (yet) compatible with the device, people with unlocked iPhones have sometimes chosen to cope with using 2G data to take advantage of other advantages the carrier might have. In some cases, this may included unlimited data as cheap as $5 per month.
In order to compete with AT&T, Verizon, and Sprint, T-Mobile is in the process of upgrading its network to support the 1,900MHz band of HSPA+. That should allow it to carry virtually any phone supported by rival GSM/HSPA+ provider AT&T, and more specifically, the iPhone. It's uncertain if Apple and T-Mobile are in negotiations at this stage, but the latter could start carrying the iPhone as soon as this fall, when the next model update is expected to ship. The carrier has promised to have the 1,900MHz band up and running by the end of 2012.