The folks over at the N-category Cafe are revisiting the idea of having
a math wiki. This seems
to be a perennial issue for us in FRIAM, and the N-Cat discussion on
this issue has some interesting angles
vis-a-vis Wikipedia, so go there and read:
http://golem.ph.utexas.edu/category/2007/09/towards_a_higherdimensional_wi.html
Carl