Try these:
a) Create a new user. Then log in as that user. Does it still do this? (it probably will)
b) Create a new network location. Does it still do this (it probably will, but there's a possibility this could make a difference)
c) Boot from a tester OS. If you still have the original HD, put it back in, and boot from it. Does it still do it?
If it still does it after all three of those, I'd be highly inclined to think it's the airport card itself. To know for certain, someone would have to put a new card in, and at this point, I'd say bring it in for warranty service.
If any of the things above does make a difference, then you basically have a software issue somewhere, and the level that makes a difference tells you roughly where to start looking. The symptom sounds like a card failure, so I doubt any of the above will make a difference, but a proper diagnosis can't be made without trying, and you could just skip all the way to the last test--a known-good tester OS.