OK, I had this when I first switched to a BT homehub, and I eventually tracked it down to the network manager. I removed it and installed wicd instead. Since then, no problems reaching the network. Maybe try that?
Edit: First - before you do that, install the wireless-tools package and run:
sudo iwlist scanning
This should tell you whether or not your wireless adapter is actually seeing the homehub.
Maybe also post the output of