If you would like to use the video chat feature in iChat AV with someone but only one of you has a webcam (or microphone for that matter), then you can control-click on a buddy and select "One-way video chat" (or "One-way audio chat").
[robg adds: I had overlooked this feature entirely until someone pointed it out to me. Now I use it to send video feeds of the new baby to my camera-less relatives.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030626065416727