You didn't read it here first, but it's worth noting: Nextel has unique technology that lets them serve businesses who need push-to-talk quite well, but it's a mess of spectrum and their next-generation data plans are all on the drawing board. Sprint needs to be bigger, and wants the business audience, plus they need to roll out PTT. But they have a 3G plan that's committed to. Could be a decent merger of equals or thereabouts in terms of revenue, subscribers, and what they're bringing to the table.
If the merger happens, T-Mobile becomes the distant fourth player in the market. Meanwhile, it's also possible Verizon Wireless will buy Nextel. Verizon Wireless is the only other CDMA carrier in the market now that AT&T Wireless customers as part of Cingular are either on GSM or required to move.
Om Malik has some of his usual great analysis of the matter, including the merger's impact on Flarion (the next-generation non-standard wireless data technology Nextel was trying out) and Motorola (which sells Nextel quite a few handsets).