Processing.js v1.0 Released
processingjs.org
processingjs.org
How much of it works with excanvas in IE <9? http://code.google.com/p/explorercanvas/wiki/Instructions
Also, canvas is just one of the html5 features it uses (there's e.g. also audio).
However ultimately I don't see what the big advantages of using processing.js are compared to plain js with html5?
I think that is also what the website says, but when I experimented with both it didn't seem like drawing shapes/blitting bitmaps on canvas with js was all that different from doing it with processing, although I didn't try doing any animations.
One downside that turned me off about processing is that the available tools (IDEs, debuggers, profilers) are even worse than what js has - you basically have to use the js tools on the generated js.
Another down side is that the language is less powerful than js, but then again you can also code in js and use processing.js as a drawing library, which alleviates both issues.
It's a lot easier to see the point with java/processing, since java2d is quite a bit harder to work with (or rather, it has many more gotcha's) than js/canvas.
Personally, I use processing.js because I can use all the existing Processing examples as a point of reference. It is easier to get the ball rolling as opposed to starting from scratch.
As a plus, the experience I get with processing.js, I will be able to apply to Processing (as you probably know, Processing is often used to graph output from an Arduino board, something else I have as a hobby).