Sony Ericsson Xperia X10 :: Other Downsides To Getting AT&T Phone Over Import / Unlocked Model?

After three years of iPhones, I'm all set with that ecosystem and I want to move on the Android. I followed the Rachel UI since it was demoed last fall and have had an interest every since. Now that the phone is finally in the US I have a couple of questions:

1. Why is AT&T's No Commitment Pricing $349.99 while Sony Style's is $499.99? Neither price involves a new contract or extension.

2. I've read something about Android devices on AT&T being limited in terms of apps. Does that mean I can only install apps from the Marketplace? If that's the case, is it that big of a deal? I've never felt the need to jailbreak my iPhone, for what that's worth.

3. Other there any other downsides to getting the AT&T phone over a import/unlocked model?

4. Anything else an iOS convert should know?

I've poked around on these (and other) forums but couldn't really find black and white answers to my questions.