Meetup does something along these lines. Check out the revision history and first paragraph:

http://www.meetup.com/terms/

Tumblr actually does this already -- they actually put them on Github itself, e.g. https://github.com/tumblr/policy/commit/35a9f50f9e8b1f419057...

You should also check out the excellent http://tosdr.org/ project.

Sounds like a good project idea... TOSdiff.