I've got my N900 running on AT&T and they definitely don't know that I have a smartphone, so I've still got the old-school 15 dollar unlimited edge connection. Slow, yes, but I'm a cheapskate.

I did get a Nexus One for my sister and AT&T automatically (and forcefully) updated her to a newer smartphone data plan.

I always assumed they had a database of IMEIs, and reading what chris_r says it totally makes sense. AT&T probably doesn't have N900 IMEIs.

(EDIT: grammar)