GPS not working

Try this -

1) Settings > Enable 3G Change to Off.

2) Use the iPhone GPS receiver to locate your position

3) Settings > Enable 3G change to On.

There are two positioning systems in the iPhone 3G, one is the 3G network, and the second is the GPS receiver. The problem occurs when the data from the two systems doesn't match.

Make sure that you are outside with a clear view of the sky, this is not a must, but it can help. Go to setting, general, now turn on the location service.

I found this on the web, it worked for me, give it a shot -

Rebooted. 3G on, WIFI on, connected to AEBS: Actual location: Memphis, TN, iPhone says Stamford, CT, with no GPS lock.
Turned 3G off, WIFI on. Location is within the right area now. No GPS.
Turned 3G off, WIFI off. GPS works.

Hope that your iPhone 3G finds you :-)

Following JhonF's advice please check that you have your location settings established:
Settings / General / Location services set to ON.