If AT&T keeps the iPhone, and not even GSM-friendly T-Mobile gets to have it, this will likely be the longest exclusive of any phone on a carrier in history. Over 3 years of exclusivity is extremely damaging to the consumer.
Take France as an example, thanks to competition with the major 3 carriers, the 3GS dropped in price after just 2 months of being out by nearly 50 euros across the board, and so did plans--adding unlimited texts, or friendlier calling plans (they work very different in Europe). All thanks to the Orange monopoly being crushed.
If you just LOVE AT&T and want to stay with them, then that's GREAT for you. But having the iPhone available elsewhere means that you AT&T bill will likely go down thanks to the competition. So even if you are a rabid Verizon hater (um, okay?) the fact Verizon would be competing with AT&T for the same customers over the same phone means that your bill could go down even if you do nothing but keep handing AT&T your money every month. You win too.
Competition and consumer choice should always be the goal. And for those hoping AT&T gets to keep an exclusive phone for over 3 years--it's nonsensical, illogical, and unless you like overpaying, bad for you.