These e-mails from AT&T are almost always smoke and mirrors. I'm on the same data plan I had in the Cingular days and have received dozens of e-mails and texts warning me that I "may be violating my contract" and that they're going to switch me to the $60/mo plan.
I've yet to be switched away from my $10/mo data plan.
I was using an HTC TyTn2 that I didn't purchase at a Cingular Store and I was on the $15 data plan.
Never got caught with this phone, probably because they didn't know what phone to map my IMEI to.
I got a Nexus One for AT&T last year, and received an auto upgrade SMS shortly after, saying that my Nexus One required another plan...
I've yet to be switched away from my $10/mo data plan.