yes, this is true. Microsoft have never supported webcams on Mac, and the 3rd party msn apps have had a hard time developing a working webcam solution for the protocol.
aMSN, which is a rather ugly Java port of Windows messenger has some basic support for webcams, as does the above mentioned Mercury Messenger for msn, although it's just downstream.
a lot of people i know have been signing up for a free .mac account just to use iChat with, and with more and more people getting macs, the iChat networks seem to be improving in popularity. that is the single best webcam experience there is on the consumer level, mac or pc.