I was able to restore 3G to my iPad, but I had to start over from scratch with my iPad settings, apps, etc.
Here is what I did:
1. I backed up my iPad (right click on device in iTunes and select "backup").
2. I restored iPad to "original" settings (which is a misnomer, b/c if you've already upgraded, iPad will restore to essentially a blank current firmware state, NOT the state the iPad was purchased in).
3. When I plugged the iPad back in, iTunes asked me to set up the iPad. I had a couple of choices: I could restore existing iPad profile, or create a new one. Here I tried to restore existing profile, but then 3G still didn't work. So I went back to step 2. On my second try, I created a new profile for iPad, and then re-selected what data I wanted synced to iPad.
When that process was complete (the second time), 3G worked.
I lost all my custom settings and application data (which is a pain), but at least my iPad is working on 3G again.
(Or maybe, according to Apple support's theory that AT&T was the problem, AT&T decided to re-broadcast a 3G signal at the exact same time as I restored my iPad.)
This was clearly a software issue, and it had something to do with my profile. It was a setting (or something) that got backed up with my iPad profile, so I had to start with a new profile with a clean slate of settings in order to get 3G back.
I don't know if that will work for anybody else, but thought I'd at least share my experience so you'd know how I got 3G back on my 4.2.1 iPad.