I don't have an answer, but I do have a related question that I've been wondering about. Is 'one' impression counted for mulitiple ads on a page, or does each ad on a page count as an impression... ie, three ads on one page, one hit = 1 or 3 impressions?
doesn't depend on the number of ads on a page. i have websites with three ads on a page and others with just one, and do a fairly detailed job of tracking traffic. if i visit both websites (one w/3 page views and the other with one) and then look at google traffic report immediately afterwords, both only show one more page impression. visiting different pages while on a website will result in more impressions however. and yes i would say adsense is accurate for impressions...for me it has been at least. i have used another hit counter as well and it shows about the same traffic as adsense does
I've found it pretty accurate, but you don't get all the cool stuff like with google analytics. Of course, if people are blocking ads or scripts, you're not going to see it in GA, or in adsense. The most accurate way is to look at the server logs.