I know there is software out there, I looked into this a little while ago. This site sells a cheap program, $10 I think and has a demo to download.
http://www.ecamm.com/mac/ichatusbcam/
Also there is a program Macam that I have used with an old logitech webcam and yahoo instant messanger, I couldn't test out iChat because I only have 10.2.8. You can find that program here:
http://webcam-osx.sourceforge.net/ . There are several different versions you might have to play around with and test out. Also check this site:
http://www.ioxperts.com/
Some logitech camaras are supported, some aren't so it depends which you have. Hope that helps.
Andrew