Welcome to the forums.
I'd check out the coverage maps on each of the carrier sites. They all have unlimited data plans, although some aren't truly unlimted (I think Verizon caps at 3G/mo, not sure about AT&T).
I believe T-Mobile has the cheapest Blackberry data plans, so that may also be a consideration if you're about to graduate.
No longer a BES Admin, but it was fun while it lasted!