That would actually be kind of cool if they kept T-Mobile as UMTS, Sprint as CDMA, shared WiMax & LTE between both (no real reason NOT to), and allowed you to move your contract between the two networks if you wanted to buy a new phone that didn't work with your current network.
It would be ironic, in a way. T-Mobile's ancestral company was actually launched by Sprint after the original PCS spectrum auction in the mid-90s. Sprint ended up owning more spectrum than they knew what to do with, so they launched Airtouch or Voicestream with their surplus spectrum. That's why T-Mobile's data sucked so badly until recently -- they literally had the bare minimum bandwidth in most markets to operate a viable GSM voice network (it had to be a VIABLE company, but they wanted to make sure it would never be a serious COMPETITOR). Were it not for the Airtouch-Voicestream merger into T-Mobile that somehow picked up surplus spectrum from AT&T or Cingular (I think the spectrum's sale to T-Mobile might have been a condition of AT&T and Cingular's merger), they wouldn't have had enough spectrum to launch EDGE, either.
Even more ironically, with the combined spectrum of both Sprint and T-Mobile and a tiny bit of shuffling that at this point probably wouldn't even be noticed by users of either network, Sprint could LITERALLY offer UMTS at international 1900/2100 frequencies in most markets -- almost certainly as a premium-rate service for $10/month extra, just because people who want a cool European/Asian imported phone or who are visiting will pay it because they have no other choice.
The original reason why T-Mo had to do UMTS at 1700 uplink was because T-Mo didn't have enough 1900MHz spectrum in most places to handle BOTH GSM voice traffic AND UMTS. If they pulled off a 1900MHz channel to use for UMTS uplinks, they literally wouldn't have had spectrum left for their GSM voice users. Sprint, however, has 1900MHz spectrum to burn just about everywhere. They could use a chunk of Sprint's 1900MHz spectrum in most cities for UMTS uplink without giving it a second thought.
That would literally give Sprint (and its customers) the best of all worlds... a network that can do both GSM and CDMA2000 voice, UMTS and EVDO (not to mention 1xRTT & EDGE), plus WiMax now & LTE later. It would also make them the de-facto roaming partner of nearly every carrier on earth -- CDMA or GSM, because they'd be the only company in America with the spectrum to offer UMTS roaming on international frequencies (without a merger, no one company owns the necessary pair of uplink/downlink channels, and no company would be crazy enough to sell the channels that would instantly make the buyer a much stronger competitor to them). It would be good for T-Mobile as well, because it would give them a viable 4G future that wouldn't require tearing apart and trying to repurpose half of the 3G network they haven't even finished building yet.
but if it means more 3g and possibly 4g and that I can keep my current plan, well bring that Nascar wallpaper on than