My friend and I have been thinking about this for a while.
Q.How would people share notes?
A) Could be live sharing in class (etherpad for ex: http://piratepad.net/sj8l1FIUIK ), then contents of pad get transferred to a wiki and people are free to edit.
Q2.Why would people put in the effort to type-in their notes?
A2) Therein lies the trouble...
If you can think of a business model, or an incentives structure that would make this a open collaborative project, do pm me. Note: It would be against my values to put ads all over site to monetize.
In statistics in college, a I and a bunch of my friends (CS students) took cooperative notes in Google Wave (which, by the way, I still maintain was amazing). Generally, a friend would be taking notes as quickly as possible, and I would follow him and replace his ad hoc notation with the correct unicode mathematical symbols to make everything look really nice. But then we found all the homework solutions and sample tests (by looking at the format of the URLs of the previous ones he gave us).
When I was in Germany, a group of students (who wanted to do it) were given the task to take notes in latex. Then every week they compiled the pdf and was put on the website of the faculty.
To write the latex file, they worked together (sharing the same file, same idea as google docs).
And at the end, they put the effort only because they thought it was nice to have all the notes in pdf, commitment with the class, and because it was a good idea to share it with the other students. I still have all my mathematic classes pdf's.
I used this site a few times in college - http://www.notehall.com/ . The downside is you have to pay for notes, study guides etc. but they let you preview the notes and then leave reviews after you've purchased.
