I am having the same problem. Ever since I upgraded to 2.2, my 3G signal is worthless. I just got off the phone with Virgin Mobile. She walked me through some sort of reset process where I had to dial ## and then some numbers. Supposedly, that's supposed to fix the problem after 4 hours.
But I've also been having a problem getting a WiFi signal at my workplace. My WiFi at home works fine. And the WiFi at work on my iPad works fine too, so the problem appears to be with my phone. I told Virgin Mobile and they said that if this reset process does not fix the problem, that I may qualify for a replacement phone.