Recently I had a problem where my system crashed when trying to wake it up from sleep mode. I was running Mac OS X 10.0.3. After rebooting, my AirPort card could not be found. It did not appear to enumerate within the IORegistry, beyond the "radio" object.
After upgrading Mac OS X to 10.0.4 via Ethernet, it still did not work. The problem was solved by removing /var/db/SystemConfiguration/*, rebooting, and reconfiguring my network. I did not bother trying to see what specifically caused this, I just started from scratch.
Hopefully this can help somebody else.
[Editor's note: I'm not sure how many people are still running an older version of OS X, but if you are, the above hint may be of assistance.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=200205140011592