I spent too long getting a simple thing working. In the end it turned out to be a mistake in variable scoping (sigh).
But I have something! You draw rectangles on the canvas with your mouse (gasp!).
See the example at : http://www.goerp.nl/js/apptest1/default.htm
or an update at update: http://www.goerp.nl/js/apptest2/default.htm
Now it’s a lot of squares floating around eating each other, dieing etc.