About once or twice a day, the strength indicator on my AirPort drops right down and eventually disconnects. Needless to say, this is pretty frustrating, but after four months, I have finally discovered the reason. It is because Bluetooth uses the same frequencies as 802.11, which opens up the possibility of packet collision. When this happens, Bluetooth is supposed to hop frequencies, and 802.11 is supposed to add a delay somewhere to compensate.
To fix, simply turn off Bluetooth, turn off AirPort, turn on AirPort, then turn on Bluetooth. This also usually fixes the reverse problem, where there is lag with a Bluetooth keyboard (and possibly mouse, but I've never used one).
I should add this is probably only a problem if Bluetooth is communicating continually. In my case, the problem is my keyboard. If you're just syncing your phone or something of that nature, then this will probably not affect you.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050816120926231