(Okay, I can't make head nor tail of how to post a non-treed response - this is why I never use Google Groups anymore)
Attack still going on. These are NOT actually users, there is no mouse movement to track. It occurred to me that (putting on a tinfoil hat) it could even be someone ticked off enough at Google to want to render a bunch of Analytics data useless - this being the test. We are seeing more and more reports of this on WebmasterWorld as it spreads. In my case, there were no CPM banners on the home page, just AdSense, which was immediately removed once the traffic started. Nor do I think I am being targeted myself - I have many sites, and if someone really wanted to target me, they'd pick one of the bigger ones.
I'm really thinking infected Windows botnet. But what the purpose is, I do not know.