On Friday, a number of Google AdSense publishers reported that AdSense was down. The cause was unknown and Google officially never actually volunteered more information about it.
Google AdWordsAdvisor was able to provide the publishers with information -- mostly ensuring them that Google was prioritizing the issue -- but there is no further information on behalf of Google's actual AdSense team.
AdWordsAdvisor does say, though, on a completely side point, that "In the AdWords side of the world, though, the stats that advertisers see in their account are delayed by up to three hours as a matter of course, under normal circumstances." We're not entirely sure how this factors into reporting on Google AdSense's side, but this is interesting information regardless.
Forum discussion continues at WebmasterWorld.