I think all of these are basically really cool, and am familiar with their obvious differences (SVG vs. canvas), but i'm unsure of their underlying differences, advantages or limitations in terms of development time, performance, and support.
I experienced Processing as a framework more suitable for sketching, prototyping experimental visualisation projects.
Raphäel is a very clean library on top of SVG. But you are bound to graphics only with Raphäel. No HTML elements and no nested layouts inside of it. But it's very easy to understand how it's built up, to learn it and best of it - it's in active development. Raphäel 2.0 came out march 15. 2011 and has some nice new things to it: http://www.raphaeljs.com/2.0, source: https://github.com/DmitryBaranovskiy/raphael/tree/2.0
Don't know Cake, so I wont go into that one.