Custom APN for iphone 3G OS > 3.0 (for me Airtel India)

First enable Custom APN: http://www.troublefixers.com/how-to-enable-gprsedgeapn-settings-in-jailbroken-iphone-3gs/

Now go to: Settings->General->Network->Cellular Data Network->

Change APN of Cellular Data to your APN (Airtel: airtelgprs.com).

Restart