The IMEI number is a number that is set by a standards group and specific series of these numbers are assigned to vendors. Given a IMEI number "anyone" can find the manufacturer and model by entering the IMEI in sites similar to this one http://www.numberingplans.com/index.php?page=analysis⊂=imeinr
So, most likely, AT&T knows exactly what phone you're using on their "Data" network based on your IMEI.
They recently passed a policy that requires "Smarphones" like the N1 to have a data plan. Now, what's not clear, is if AT&T will allow you to have that phone in their network just for voice. No data whatsoever. See Data Plans and Smartphones in their terms and conditions here: PLAN TERMS - Wireless from AT&T.
Given the current animosity between Google and Apple and the tight relationship between AT&T an Apple... the fear was that AT&T would cripple or disable the N1s (specifically) when AT&T identified them on their network. This was "speculation" based on Apple pressuring AT&T to do so given that Apple has an "exclusive" contract with AT&T that prevents Apple from offering the iPhone on other networks. "Maybe" this "exclusive" contract prevented AT&T from offering a Smartphone with features similar to the iPhone. (The iPhone is AT&T cash cow and AT&T has not offered a Smartphone that challenges the iPhone's features) AT&T, so far, has done nothing to cripple N1s given their existing policy on unlocked phones shown here AT&T Choice
under Can I use my non-AT&T phone.... They "might" never do anything to affect the N1 as Apple might only have a say on Smartphones offered/supported by AT&T. We all know that AT&T does not "officially" support the N1 on their network.