The weirdest thing happened the other day:
I completely lost Bluetooth on my Powerbook (15", 1.5 GHz) while trying to sync with an unsupported phone on Tiger 10.4.2.
Booting from an external drive and visiting the System Profiler showed "not available" on the Bluetooth interface, and the Bluetooth System preference had disappeared, while the Bluetooth menu bar icon showed the broken line and the pulldown menu read "not available" - again, while booted from an EXTERNAL drive (running 10.3.9). Bluetooth file transfer refused to work either, citing lack of Bluetooth hardware.
Resetting the NVRAM via Open Firmware didn't do anything, either.
Today, I hook up a D-Link USB Bluetooth adapter to the Powerbook, and the Bluetooth preference shows up again, everything works fine.
The odd thing is, it still works fine even AFTER I've long UNPLUGGED the Bluetooth adapter.
System Profiler shows perfectly functional internal hardware.
(I mean, I'm happy that it works again, but wtf happened?)