Yeah, it's not so much fixies as it is hard gearing+low cadence.
There's a lot on the subject...
http://yamabushi.wordpress.com/2007/...or-your-knees/ it's a good article.
Fixie skidding is definitely bad for your knees.
When you're looking for peer reviewed articles on the subject, don't look for "fixie" stuff proper, rather check out cycling and knees, gearing and knees, etc.