After one of the recent 10.4.10 updates on an Intel iMac, the microphone on my Plantronics USB headset appeared to be disabled. The solution that worked for me was to use Applications » Utilities » Apple MIDI Setup, and access Properties For » Plantronics Headset under Audio Devices. At the bottom of the panel, there is a Mute checkbox, and unchecking this restored the microphone's operation.

I can't take credit for the fix; I found it via Google on macspeech.com. However, I thought this might be worth passing on to macosxhints.

The 10.4.10 update did not cause this problem on a G4 PowerBook, so I believe this is related to the recent Intel audio fix that Apple released. However, after launching Apple MIDI Setup on the G4, the microphone was disabled and the checkbox was not accessible. The only solution there was to unplug and re-plug the headset's USB cable.