You can, there are several programs that make your screen show up as a webcam. Also, we're currently working on getting VLC support up and running.

Is there anything currently available for the Mac?

Googling led to this for Windows: http://www.pcwinsoft.com/screencamera/landing.asp

yes, try camtwist:

http://allocinit.com/index.php?title=CamTwist

Here's a great video tutorial made by CamTwist explaining the basics to using it: http://www.youtube.com/watch?v=0uMUTvI3NAo

CamTwist is fantastic for screencasting on a mac. Works great on http://ustream.tv

Which is the better service for live demos? Are they both free for commercial purposes?

You should try both and see what you like best. Mazy works for Ustream, I work for Justin.tv.

How about Linux?

ucanvcam should do the trick: http://code.google.com/p/ucanvcam/