If only there were an API for mouse capturing. Is there any reason why it couldn't work if it were implemented like full screen mode (displaying a message telling the user to press escape to reclaim the mouse)? Edit: Actually it looks like this is being worked on after all – http://dvcs.w3.org/hg/pointerlock/raw-file/default/index.htm...
I just implemented that in a fork: https://github.com/danielribeiro/WebGLCraft/ Only works in Chrome for now, if you enable it in chrome://flags. You don't even need to be in full-screen mode, at least on the latest version. Pull request is here: https://github.com/danielribeiro/WebGLCraft/pull/1
Page at http://ricardobeat.github.com/WebGLCraft/, press C to enable mouse lock. Aiming doesn't work very well...