"This morning, we temporarily took down the Facebook site to fix a bug we identified earlier today," the company statement read. "This was not the result of a security breach. Specifically, the bug caused some third-party proxy servers to cache otherwise inaccessible content. The result was that an isolated group of users could see some pages that were not intended for them. The site has now been restored and we apologize for any inconvenience this may have caused."
That would be why, it seems, some Facebook users (both on blogs and in our comments) reported that they had seen what appeared to be other users' data on their Facebook log-ins. So, to the bloggers who speculated a proxy issue: looks like you were dead on.