Airport connection on Macbook

My son is having a strange problem with his macbook. It was working fine and sometime last week (he didn't tell me exactly when), he lost connectivity to the home wifi n/w through Airport. We have 4 other macbooks in the house that are working fine so I know it's not the n/w that's at fault. Just in case, I've rebooted the router/modem/etc. He swears he didn't change anything or install anything and an examination of the system seems to support this assertion.

Here's how it's acting right now: Network diagnostics show that it's seeing the n/w (and neighbour's networks) and show's that the Airport and Airport Settings are working (they're green) but the Network Settings are yellow. I've no idea why.

Strangely, he took it to another location and he said he was able to connect to their n/w fine.

I've tried the Ethernet connection and it works fine.

I reinstalled Snow Leopard just in case as I feared something got mucked up but that hasn't helped.

I ran the complete h/w diagnostics and they came back clean.

I'm at a loss as to what to do next.

Ideas?