I have had the same problem as well. Some random person hit my site up with 40 clicks after I got a few links from other sites. My account was terminated a few days later. Google gives the opportunity to appeal, but I chose not to do so. This also means that if someone were to have something against a site that runs AdSense, they could effectively shut down the source of income by mass clicking over a day or two. I was considering running a story about this on a few sites, but in the end Google can do as they want with little or no challenge. Google the next Microsoft? Maybe.