[Nexus S Q&A, Help & Troubleshooting] stevelion:
Supposedly AT&T detects a Smartphone based on the IMEI number of the phone matched up to a database of IMEI numbers of phones that AT&T sold. The latter is a very important point as they did not sell the Nexus One phone, therefore, many argue on various forum posts that AT&T cannot recognize this non-AT&T sold phone as a Smartphone. The same would be true of any phone not sold by AT&T. However, I have yet to come across any posts of anyone that actually owns a non-AT&T sold smartphone who can confirm this and I've been searching far and wide for this confirmation. I'm getting ready to buy a non-AT&T branded unlocked smartphone to try this myself so perhaps I'll be able to confirm. It would be great if the Original Poster provided his experience with this. i.e. did AT&T detect this phone via the sweeps?

To address some other questions and comments in this forum post...keep in mind that AT&T required a data plan for all smartphones activated on its network after September 6, 2009. However, this policy is far from clear and there are many gray areas, this being one. For example, if I had a smart phone active on September 5 with no data plan, and I buy a used Nexus One on eBay and stick my SIM in it, do I need a data plan? I didn't purchase a new phone or "activate" anything on AT&Ts network so I would argue not.

I also not believe it matters if data is enabled on the phone or not. Try this...take the SIM out of your AT&T phone, stick it in another (different) phone, make a phone call, go to OLAM (i.e. OnLine Account Management), and look at the phone that AT&T thinks you have. Data enabled or not, if the phone was sold by AT&T, they will have identified the phone you just used.

Here is another gray area...data plans on a non-Smartphone lines were as low as $10/month. Data plans are enabled the on the line (i.e. the SIM) not the phone, so a SIM with a data plan can be moved from phone to phone (e.g. dumb phone to smart phone) and data will be active so long as the phone supports the active APN (Access Point Name). If I have a dumb phone line with active data and my phone breaks, what if I stick that SIM in a smartphone I have laying around so that I can make a phone call? What is my obligation to contact AT&T and notify them that I need smartphone data plan?

What follows is for reference only for those that may want a deeper understanding of AT&Ts data plans or desire to "experiment..."

AT&T uses two primary APNs: wap.cingular and isp.cinguar. AT&T is now using wap.cingular for dumb phone access and isp.cingular for smart phones access meaning, if your line has dumb phone data active, your phone needs to support the wap.cingualr APN. Most smarter phones allow the creation of custom APNs if it is not built-in.

The login an pw for the two APNs is as follows: wap.cingular is login= wap@cingulargprs.com and pw= cingular1. isp.cingular is login= isp@cingular.com and pw= cingular1.

The ability to enable the various data plans and features on your account is dependent on what type of phone is assigned to your account based on the IMEI number assigned to that account. This is not the same as the automatic detection I mentioned above. The assignment is done by AT&T employees. I do not have exact or inside information, but I surmise that your line is associated with one of about 4 primary phone types: smart phone, iPhone, black berry, or dumb phone. This assignment dictates what data plans and other features are available to you either thru OLAM or via a CSR. For example, if your line is identified as a dumb phone, you will see the dumb phone data plans in OLAM. If you currently have a smart phone on your account, and switch to a dumb phone, you will only see the dumb phone data plans if AT&T switches the assignment to the dumb phone IMEI. This is only done by calling AT&T, not merely by making a phone call from the dumb phone and relying on the automatic detection.