I've got an issue with mine, or it could be BT. I connect to an open zone network at work, and if I log in using my BT broadband account then leave my phone locked for a while, it will lose connection to the network and I'll have to rejoin the network and sign in.
Is it an issue with the phone, or could it be that the dhcp lease is set to expire after a short period of time, in which case I'll have to suck it up?
Edit - also, it doesn't pick up my home wifi when I get home, I have to go in to settings and select it (it remembers the password).