Re: the second paragraph. You have the same question, in spades, for the data. Why trust the data? Whoever put it on the web undoubtedly had a reason for wanting to share the data; assuming that they're pure of heart and simply want to expand the world's knowledge pool is problematic.
As far as computation goes... two thoughts spring to mind. The first is that SETI@Home and Popular Power had the same question-- how do you prevent cheating. They came up with two solutions: (1) occasionally send the task to multiple users and compare answers and (2) have time and length estimates, that you can use to doublecheck that the time and length claims made by your cpu source are accurate.
The other thought is that it's often a lot easier to verify answers than come up with them. Mathematical proofs are an obvious case of this, but so are things like travel plans (and J2ME's new class verifier works on this principle).
I was thinking of the Google situation. It would be much more efficient for website owners to add their own metadata than for Google to slurp up the entire Web and try and work it out. I'm sure website owners would be very keen to do this -- they would, and they do, lie through their teeth.
I think your Seti & PP are particular kinds of examples where they already have the data. So it is interesting from the point of view whether it is cheaper to ship the data somewhere else perform your own computations, but doesn't really have much to do with the semantic web, which as I see it, is much more about some form of machine understanding of data so that some undetermined alaysis can be performed on it. Specifically, analysis by people who don't create or control the data.
I think your Seti & PP are particular kinds of examples where they already have the data. So it is interesting from the point of view whether it is cheaper to ship the data somewhere else perform your own computations, but doesn't really have much to do with the semantic web, which as I see it, is much more about some form of machine understanding of data so that some undetermined alaysis can be performed on it. Specifically, analysis by people who don't create or control the data.