This really came out of a throwaway comment I made on Daniel, but it seems popular to pick apart every last iota of Steveness from the WWDC keynote, and I'm nothing if not popular. So here we go.
What is WebClip? In fact, that's not really the question I want to be asking. We know what WebClip is; it's a technology which lets users see only the bits of web pages that those users want to see. The real question is what does that mean? Well, I know which bits of a web page I usually want to see; they're the bits which aren't adverts.
I'm going to go out on a bit of a limb, and guess that the way WebClip works (I'm not a WWDC bod so I don't have any more access to the new stuff than anyone else; in fact I haven't even downloaded the Safari 3 beta) is by observing which DOM elements are within the clipped region, and downloading only media relevant to those elements. If that's the case, then you can ignore the fact that the ads on the page don't get seen; they don't even get downloaded. Therefore if I'm reading, say, the Dilbert strip in a WebClip, I'm effectively getting free Dilbert, even more free than the free website because I'm not upping their ad impression count.
One thing I noticed about the various sites that Steve clipped is that as far as I can remember, none of them features 'Ads by Google'. It would be quite embarrassing for Apple's CEO to demonstrate how to reduce revenue for one of Apple's most prominent board members in a world-broadcast keynote talk. As over 99% of Google's revenue is from online ads, and Eric Schmidt (CEO of Google) is on the Apple board, that is exactly what Steve was showing us, though. There'll be a doughnut fight back at Infinite Loop over that, I expect.