There is an interesting post by Jeff Croft on online typeface choices, and what we could possibly take advantage of.
I don't think I've ever entered in Myriad, or Century Gothic as a type choice before, but yet I think it's been installed on every machine I've ever worked on. Same goes with Futura, American Typewriter, and so on.
He did find some reference material that details the percentage of installation based on platform. (Windows, Mac, Unix) and the Visibone Font Survey.