This is what the e-mail said that I got from AT&T:
Weird thing, I had the 2G data plan.. So IDK.. I didn't want to fool with AT&T to much.. an extra $10 a month isn't that bad... (even though I dont have 3G service <_<)
-- Added in VIA Edit --
Also, I checked online when I got this email to see what phone was registered with my account. Sure enough, it had changed from "Apple iPhone" to "Apple iPhone 3G".. So either they keep track of this stuff VIA the IMEI like Flhusky said above.. Or Apple snitched to AT&T, or the guy I talked to about a "hypothetical situation" changed it over... I'm going to go ahead and assume they deduced it was from my IMEI.. Even though when I called to upgrade my plan, they had the wrong IMEI on file and it wouldn't let the rep upgrade me to the 3G plan... So, I guess that does leave someone snitching :-\.. Oh well.
Even thought you don't have 3G service, you still have to pay for the 3G plan because they can't keep track of when you're on 3G or not. You have a 3G phone, so therefore you pay for 3G service. Hmm, I wonder if Apple would come out with NEW iPhone that has GPS, but no 3G radio.... They could use the plastic design of the 3G, but call it just the iPhone.