Only a hard restart (holding the power button until the machine shuts off) can revive it. The issue seems to be more prevalent on machines running Mac OS X 10.6.3, but some 10.5 users have spotted it as well.
In long-running thread (12 pages worth), the affected users have identified a few potential triggers: The Sudden Motion Sensor, overloaded virtual memory swap files, and an excess of remembered Wi-Fi networks. After disabling the motion sensor (how-to here), several users reported that the issue all but disappeared. It wasn't eliminated entirely for everyone that tried it, but it seemed to occur much less frequently.
Another group reported a sharp decline in the issue after cleaning out the machine's list of remembered networks. To do this, launch System Preferences and click Network. Select Airport in the left hand column and then click Advanced.
A new slip appears with a list of your "Preferred networks." This is a listing of all of the Wi-Fi networks you've ever successfully connected to. If you're the type who hops from coffee shop to library to bookstore, it could be quite long. Simply select any you'd like to eliminate and click the "-" beneath the list. Just remember that, should you encounter those networks again, you may have to re-enter your access info.
My MBP is an ancient artifact, and so far free of this issue. Now, the inevitable question: Has this trouble plagued your machine, and if so, have you found a fix?