March 10th or so, Google stopped updating the site performance reports in Google Webmaster Tools. Googler, "sreeram" said they are "making some changes to the algorithms and pipeline," and thus had to pull down the tool for a bit. He hoped it would be available within a few days, but it seems like the report stopped running or updating since then.
A new Google Webmaster Help thread has sreeram saying that he hopes to bring it back online by the weekend. He said:
No, this is an issue with our data pipeline. We are actively working on it; updates should resume by the end of this week.
Many webmasters take these metrics and reports very seriously, so I hope it is up and running, accurately soon.
Forum discussion at Google Webmaster Help.