I think what AT&T might be doing is seeing that you have an iPhone.

Since the iPhone 3G you have is being registered on AT&T's network, AT&T identifies it as an iPhone and automatically signs you up for their data plans.

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iPhone 2G T-Mobile USA
17 November 2007
1.1.1-->3.0 (jailbreakme.com, anySIM, iJailbreak, ZiPhone, PwnageTool, QuickPwn)

iPhone 3G AT&T/China Mobile
5 January 2008
2.2-->3.1 (yellow/red/ultrasn0w, PwnageTool, QuickPwn)

I can try to answer your iPhone related questions! follow me at @acwan on twitter!