Same thing happened on my LG phone. Finally resolved it yesterday with these steps:
- Turn off "data connection" unders settings-cellular
- Turn on WiFi and connect to a WiFi network
- Do whatever it was that you could not under 3G. In my case, sync'd my Office365 email and downloaded an app from the Marketplace.
- Turn off the phone, remove battery.
- Reinstall battery, turn on phone.
- Repeat steps 2 and 3.
- Tun the cellular "data connection" back on.
So far, everything is working as it should over both 3G and WiFi.
Hope this works for others!