Oh, what a nice way to begin the new year: apparently, all incoming mails are tagged with FH_DATE_PAST_20XX, meaning:

  The rule matches the year (a string of four numbers) in the Date header,
  and checks if it is between 2010 and 2099.

...which is a silly thing to do in 2010 :-) This has been fixed upstream a few months ago, our distribution of choice is already on it.