No 3g network with Virgin Mobile iPhone 4s

I have a new iPhone 4s that I'm using with Virgin Mobile. The phone works fine except that I can't-- ever-- get 3G networking to work. Their coverage map shows that I'm in the middle of a big 3G zone, and I've tried at several locations around the city. The phone often shows "3G" at the top, but it always switches to "o" before loading anything from the internet.

Virgin support has been useless. Since it (usually) eventually loads, they insist everything is fine.

I tried resetting all network settings a co

Has anyone else had problems with Virgin and 3G? Is there anything I can do about it?