It's rumored by some that Apple will announce that the iPhone is becoming available from carriers other than AT&T on Wednesday. That would mean you could leave AT&T while staying with Apple, among other interesting things.
If you want an iPhone in the US, you're stuck with AT&T. If the iPhone becomes available on more carriers, people have a choice. And if the phones are equal across the carriers, people will be choosing based on rate plans and coverage instead of whichever carrier has the phone they want.
This would hopefully lead to lower prices, especially for services like text messages, data and voice. There's already a mini-price war going on between Verizon and AT&T, who have both dropped their unlimited calling plans to $70. If they're both fighting for iPhone customers, one expects this to only continue.
AT&T still doesn't offer tethering for the iPhone, while Verizon is offering it on phones such as the Palm Pre Plus. If Verizon has tethering, how fast do you think AT&T would scramble to get it? Assuming it's a choice by AT&T, and not some other weird technical issue, we think it would happen pretty fast. These are the benefits of competition.
Coverage and Reception
AT&T's network might not be terrible, depending where you live, but one thing is for sure: it's stuck providing coverage for the biggest wireless data hog ever made. Verizon has a great network that really hasn't been challenged. On the one hand, Verizon might handle the iPhone with aplomb. On the other, AT&T aced our 3G test and still sucks when the iPhone is on it. Is it the network, or is it the phone?
As for the assumption that Verizon getting the iPhone would lighten the load on AT&T, well, it would be nice if half of the AT&T users just went over to VZ, but that's definitely not how networks—or businesses—work.
3G vs. 4G Wireless Data
If we don't get a Verizon iPhone this year, the general expectation is that we'll see an LTE iPhone on Verizon in 2011. (LTE is the 4G wireless protocol that both VZ and AT&T are supporting.)
But if Verizon gets it sooner, it might not have Apple breathing down its neck for a sturdy LTE network by next summer. That could be bad news for not just iPhone users, but anyone looking to get an LTE phone. Remember: 3G was available when the original iPhone was released, Apple was just too cautious to hop on board before it was ready.
GSM vs. CDMA
Verizon runs a CDMA network, which would require a CDMA iPhone. CDMA phones are more difficult to hack than GSM phones, which would be a hit for the thriving homebrew and jailbreaking scene that currently exists. Not that Verizon—or Apple—would care.
Furthermore, a CDMA iPhone wouldn't be able to travel outside of the US without a GSM radio, as most international carriers use GSM, just like AT&T. Many of Verizon's BlackBerry's offer CDMA phones with GSM radios for international roaming, so it's possible the iPhone could get it too.