3.1
If you download and install OS 3.1 you can activate the iPhone with any AT&T SIM and not be placed on a iPhone data plan.
You will however have to have the UUID of that iPhone registered with someones Dev Account.
I currently use my AT&T BB Bold SIM for both my iPhone 3GS OS 3.1 B1 and my BB Bold OS 5.0.0.100. Works great. However I have to call to get the VM. that is the only feature not working on the iPhone.
Email me if you have any additional questions or need somehelp or info, UUID activation information. Thanks.