I think you want to look at the OS. DUN is the preferred networking method with the tablets, but I really find PAN to be simple (I can provide more details if you have any questions). I think there's a wiki page on this site (or perhaps the maemo.org wiki) with a list of DUN phones known to work.

I really hate wireless companies. From their site (at least when logged in with my 2-year-old account), I see 3 options:
1. $50 - PDA Personal Bundle. I assume this is what works with 3G cards, but I thought that was $60.
2. $30 - PDA Personal. Looks like it has the same f!@#ing description as the bundle...
3. $19.99 - MEDIA MAX 200 Bundle. What I have. This is 200 text messages plus unlimited data.

I originally had a Nokia 6682, which could hardly be called a smart phone, so maybe that's why I was able to get the media max bundle? I changed the phone in my profile to a BlackJack and the price didn't go up, so I can't say for sure.

In short, I have no idea how many classes of phones AT&T has, or if the $20 plan is still available, etc. Sorry I can't say for sure.