I tried the Coursera Cryptography class earlier this year. The pace was quite brisk and the material was somewhat demanding - I'd recommend at least first year university mathematics. It was fairly well presented and quite interesting, and it showed a degree of polish better than Sebastian Thrun's / Peter Norvig's original AI class last year.

I had to drop out after about three weeks because I couldn't handle it along with my actual uni course, but I'm thinking of starting it again soon.