There is something to be said for 'user rights' here. It sounds like you're describing a situation where the webmaster for Foo.com includes the script for, say, Folding@Home on their site. As a result, all visitors to Foo.com have some fraction of their CPU "donated" to Folding@Home, until they navigate away from Foo.com. Without some sort of disclaimer or opt-in, I would consider that a form of malware and avoid vising any site that did that.
That's not to say you couldn't build a system that asked for confirmation or permission, but there is definite potential for abuse.
