Strange behaviour: Everything was fine, working with my MBP on a multi-AP WDS network of new Airport Extreme base stations; move to another floor, just next to an AP, can't see the signal, or rather I see it fleetingly, get asked for password, then it disappears. My first conclusion is that the AP must be caput, but back upstairs, where I started, I now see no signal.
Another laptop can see the signal fine. I try manually changing network setup, restart, shutdown, even taking out the battery then restart. Still no wifi connection. I conclude that the wifi card in my laptop must be damaged, or at least have a dodgy aerial connection.
Back at home I still have the same problem—and neither Airport Utility, nor iStumbler can see the signal either. I think I'll try looking at the wifi card.
To check on how to do that I need to get online. So I find an ethernet cable and connect a LAN port on one of my base stations. I'm now online, as expected, but I didn't expect to see Airport Utility and iStumbler burst into life just because I made a wired connection. Everything looks normal, and Network preferences now says I have a wireless connection.
I unplug the ethernet cable, and everything stays fine; back to normal. I don't pretend to understand why.
It happened again!
Same symptoms - persisting across two different WDS networks.
Again, recovery coincided with my decision to give up hope and use a wired connection to my base station.