If you can't wait to experiment with the endless possibilities that using webcam streams with canvas can give to developers, this annotated source code (laid out by the invaluable Docco) is just waiting for you - take a look at the example usage, then download the library and have a blast!
This demo only works on the most recent versions of chrome, firefox (Aurora/nightly) and safari (possibly explorer too, but I would not vouch for it). Opera should really try to implement a requestAnimationFrame, instead, in particular now that they support WebGL.
Some browsers will ask you for permissions to use your webcam: accept it in order to see the demo in action: feel free to check the code before activating it. Spoiler: nothing will be sent anywhere, it's between you and your browser.
Note for chrome 12 users: you might experience some performance issues. If you want to solve them (and feel adventurous), take a look at chrome://flags and get freaky checking all things 2d- and video- acceleration
Here are some useful resources to learn more about HTML5 video, canvas, WebRTC and how they work together