Unlocked Nexus One ($650)
First, it's not on AT&T (I suffered through the iPhone long enough). Second, it's way more customizable than the iPhone ever was. Apple likes to make decisions for you in order to make things more "user friendly." For example, I want to be able to delete items from my desktop, but the iPhone won't let me do that. I will never check my stocks on my phone, mostly because I don't have any—but I would still have to look at that icon every day.
True, the iPhone does prevent you from really customizing it the way you want. Now that the Nexus One has multitouch (and is past all that EFT drama), I'd say it's a worthy opponent to the iPhone. Even Steve Wozniak loves it!