No server on that though is there, so no multi-user? I was thinking of Nodejs earlier and the Javascript in that example reminded me and I did a quick search.
Here's the source code for a Nodejs whiteboard, though no demo, and I haven't used it myself, so don't know what it looks or works like:
https://github.com/pascalopitz/whiteboard
Here's a non-Nodejs one, demo and download:
http://www.unionplatform.com/?page_id=2762