I was in the middle of building something like this as a demo for my XML storage server, looks like several people beat me to it. Looks like they applied for a patent only to find that there is lots of prior-art.
“A little while back I had an idea about how it might be possible to build a massive grid computer harnessing the power of the transient Internet user population. In essence the idea was to use the almost universally deployed JavaScript runtime embedded in modern web-browsers to perform small processing tasks which are part of a larger distributed processing algorithm (I can just hear those bozo bits flipping).”
James McParlane's Blog 