[Samsung Galaxy Nexus] bitbang3r:
So does GSM, as long as it's operating in UMTS mode (which it will, unless you're making a voice call on T-Mobile from an airboat 5 miles south of Alligator Alley (I-75) in the middle of the Florida Everglades & barely have a viable signal to begin with, in which case it will fall back to legacy TDMA-based 1G GSM).

They could... except then your phone would only work when uplinked & downlinked through a fixed, securely-mounted antenna. Doppler shift does terrible things to phase-based modulation. Just ask anybody who's ever tried to watch an 8VSB-modulated ATSC TV transmission during a hurricane (when the transmission antenna is wobbling) or from a moving vehicle. In theory, there are exotic antenna designs that can untangle polarized signals while moving by simultaneously receiving multiple phases & using a DSP to separate them out "after the fact", but they're *way* out of the current realm of viability for mass-market consumer electronics, and WAY more demanding than a 2" metal stub embedded inside your phone.

Truth be told, spectrum isn't the problem. Tower density is. The nice thing about CDMA is that you can literally fix almost any bandwidth problem just by throwing more tower sites at it & letting the network sort itself out like magic. CDMA has very few "hard" limits. Some, like 1.25MHz or 5MHz channel pairs, are carved in stone and can't be engineered around. Once you're in the club and own the spectrum, though, it's really just a question of "what kind of tower density are you willing to pay for. Crowded mall? Give it its own cell. More-crowded mall? Spread a dozen picocells around it, especially the food court.

Verizon is unlikely to ever support legacy GSM or UMTS directly, and can really only evolve into LTE going forward. Sprint could, in theory, buy T-Mobile, and instantly consolidate GSM/UMTS into any cell site where it has deployed Network Vision (~3% of the US, so far) as long as it had the use of T-Mobile's spectrum, with little more than a site visit, software upgrade, and some software reconfiguration. Verizon can't do that, because it ALREADY upgraded its network, and has too much in sunk costs to scrap everything and redo every cell site the way Sprint is (and MUST). Truth be told, Sprint won't do it either unless it merges with T-Mo, and the feds are unlikely to allow it (it's not 100% impossible, but VERY unlikely to happen unless there were simultaneously a merger between US Cellular, MetroPCS, Sprint's "rural" partner networks, and/or Cincinnati Bell (to preserve the status quo Quadropoly).

AT&T and Cingular switched to GSM because they had no meaningful upgrade path from TDMA. In fact, AT&T was actually planning to switch to CDMA until they bought Cingular, and altered their plans only because Cingular was already deploying GSM. In theory, Sprint+Tmo (with the spectrum of both) could semi-gracefully migrate towards GSM with backwards compatibility for CDMA2000 voice and 1xRTT (like Telus did in Canada), but NOBODY could really get away with "flipping a switch" and forcing a wholesale changeover anymore. Hell, Sprint doesn't even have enough Nextel customers left to pay the electric bill for their added tower costs, and the official iDEN sunset is STILL two years away.