Separate post for a separate comment:

I called AT&T today and spoke to a rep. I did not tell the rep that I wanted the $15/month data plan for an N9, specifically, but rather told the rep that I bought a phone abroad and wanted the $15/month unlimited data plan for basic phones. In other words, I let the rep assume that I had bought a basic phone, and did not lie to the rep directly. I also told the rep that I tried to get it in a store but that the store could not override the unknown IMEI problem in AT&T's system. The rep asked me for the IMEI number of my phone, which I gave to him - found on the N9 under Settings/About product/, so you don't need the original box when you call - and set it up for me.

All in all, pretty painless, so just to confirm for you guys - yes, the Nokia N9 will work with the $15/month unlimited data plans for non-smartphones on AT&T. You might not have to call if you already have the data plan on your line, but I doubt it. Just a reminder though - if you abuse it, you risk AT&T figuring out that you lied to them and getting cut off.