Ever since Google AdSense released CPM Based Pricing for AdSense one would assume that refreshing your page over and over again, would be against Google AdSense's terms of service. But even then, even though it says in the TOS in section 5; "Prohibited Uses";
You shall not, and shall not authorize or encourage any third party to: (i) directly or indirectly generate queries, Referral Events, or impressions of or clicks on any Ad,
People still think it is ok to fake ad impressions. A thread at DigitalPoint forums named Putting sites in autosurf? asks the question, and the first response says; "you will get impressions but no clicks. no it's not against TOS." which is wrong - you get paid for impressions and it is against the TOS.
Of course there are legitimate reasons to have robots hit your site. You may want to do load checking, stress testing, availability checking and so on. But you can easily exclude AdSense from these tests. There are also advertisers who want to boost their Alexa rankings or statistics and pay people to visit their site often. So is the next PPC fraud, CPM fraud? Or has this been an issue for a long long time? :)
Forum discussion on auto-surf programs and AdSense's TOS at DigitalPoint Forums.