iPhone N94 prototype shows native T-Mobile bands
Evidence in the device's firmware might support it being a legitimate experimental model. Visible apps preloaded on the phone are ones only available to Apple, such as the AppleConnect service and Radar.
If real, the prototype probably reflects a testbed for T-Mobile-friendly cellular chipsets rather than plans for an iPhone 4 release. Apple is rumored to be launching the next iPhone in September and would have just four months of availability even if the variant were released immediately. Should a T-Mobile version exist at all, it would more likely involve the newer phone.
T-Mobile has remained mostly silent on the prospects of an iPhone deal but has given indirect clues that it has at least been in talks with Apple. Any negotiations might be for just one iPhone run now that AT&T has proposed a $39 billion takeover that would obviate the point of a native T-Mobile version.
Apple has been under pressure to expand iPhone access to as many carriers as possible to curb the growth of Android. Verizon was considered the most important carrier with as many or more subscribers than AT&T. T-Mobile is not only the fourth largest US carrier, however, but the original carrier to support Android. It first picked up the G1 in October 2008 and has always been the home of Google's flagship phones, such as the Nexus One and Nexus S.
Update: A follow-up reminder later in the evening from 9to5 noted that the N94 was last seen using an A5 processor. The clue implies that the prototype is an early working example of the future iPhone, possibly called the iPhone 4S, that has the old body but all the internal components of what will reach the final model. Apple usually has two later engineering and design validation stages that are closer to the production edition.