Dameon has raised a very good point about the lack of Mac in the IM world, which is one reason why iChat has done so well with Mac users. Besides being interoperable with AOL's AIM and Jabber it just works.
That said, SightSpeed has seen dramatic uptake under Peter Csathy, who embraced the idea of the need for a concerted effort of outreach and has seen the growth this year.
But Yahoo is way behind in releasing a new Mac version that lets Mac users take advantage of the benefits that PC users have. AOL has not made indications of its future Mac efforts, while MSN gives it lip service.
As a Mac user we need equality.