This may be excessively philosophical (or lawyerly), but given that the basic design premise of SETI@Home and other "screen saver" software is to run at a computer's lowest priority level, doesn't that mean that it's effectively asking permission to use your computer? And isn't it also true that your computer, in the normal course of prioritizing running processes, is voluntarily granting permission on your behalf?

I'm being a bit facetious. I understand you're talking about the difference between involuntary infection by viral software versus voluntary installation, and it makes all the difference in the world whether the software got there through the front door or by breaking-and-entering.

Perhaps a more substantive question is, how would I let you opt-out of features of a Web interface that exploit your computer's processing power? The client-server model is based on the idea of distributing the load, and just by using a Web browser you're participating in a cooperative processing network. You're already doing so voluntarily; how much of your computing power can I use before you consider it excessive? When do you revoke the permission you're already implicitly giving me?

It's fascinating that more than 99% of the computing power on Earth is going to waste. An objective observer might conclude that the primary purpose of computers and software is to sit idle waiting for humans. It gets more interesting when I think about all that screen-saver-like software, including SETI@Home and similar ideas, all of it intentionally designed to live in the neglected nooks and crannies of the global computing environment. Scavengers, designed for a specific ecological niche, living on the abundant scraps of CPU cycles that fall from the table. Can you say "emergent phenomenon"? I don't expect any truly Artificial Intelligence to ever be designed, but I fully expect quasi-artificial intelligences to arise naturally...maybe out of screen-savers, of all things. But why not? Didn't we begin as pond scum? Life has humble beginnings; out of simplicity arises complexity.