Griffin Technology's iMic provides Mac users without audio in capabilties, the ability to plug in a microphone or other audio in source into their Macs.
With iChat AV, there's one more reason to need such a device -- the iMic, along with a headset/mic combo, lets you talk to others in a more familiar headset mode, rather than in speakerphone mode by using your built in speakers.
The iMic has audio-in and audio-out on seperate jacks... and iChat lets you configure the audio out to only go through the iMic -- keeping normal system sounds and music playing through your regular speakers.
Note: in order to properly utilize a headset with iMic -- it must have seperate in and out connectors. These headsets are often sold specifically for PC/Computer use.