I've been having problems for the past few days on o2 in the UK. I seem to be able to browse for a while, then pages will just refuse to load.
I found that turning 3G off and then back on sometimes helps, which I thought may suggest it's a network issue, like maybe the connection needs refreshing, but I've found today that force quitting safari (hold home button until the home screen appears) also works, so maybe it's a memory issue.
When it happens, I can't browse the web, or connect using Mocha VNC, but I can check my email accounts.