Looks like the drivers for Cirrus Logic audio card in unibody Macbook from late 2009 (also known as MacBook 6,1) are a bit broken. This is on Windows XP. I'm running Boot Camp 3.0 because 3.1's installer crashes. Anyways, microphone in Steam and Steam edition of Torchlight just didn't work. From certain posts on Apple forums, this seems to affect other software — Windows Live Messenger, et al.
Edit April 25th 2010:
Mirror of fixes: regular fix, no s/pdif out fix
Edit May 7th 2010:
After using these patches for some time in L4D2 and elsewhere, I conclude that one or both of the fixed drivers are breaking stereo support, to a certain extent. That is, apparently the "rear" speaker or even the microphone becomes the "left" speaker. YMMV.