They may not be able to detect that it's a Nexus One, but they WILL know that it's not and AT&T phone and can dig further for information and just block you or force you to a smart phone plan.
Yeah, those reps get enough training to sell you a phone and give you some support. VERY few, if any that the general public would run into, understand the way data flows from the cell tower switches to the monitoring/billing system. Unless you have actually logged into a switch and watched the files scroll through or taken captures to analyze, there is no way to really appreciate the sheer amount of data they collect instantly on everything that hits their towers. When you are driving down the road, not on a call, not using any data, your phone is constantly pinging towers to make sure it uses the best one in case you need to do something (that's how they can track your location and whereabouts, at any given time (you see them do this on CSI like shows)). The only way to keep this from happening, is to turn your phone off, and even that is tracked.