The default SpamAssassin configuration considers any date far in the future to be extremely suspicious, which is pretty reasonable.
However, as @schampeo points out, it also seems to consider any date later than 2009 to be “far in the future”.
That means that until the SpamAssassin folks roll out a fix, and that gets deployed by SpamAssassin users pretty much all email will get an additional 2-3.5 spamminess points. That’s likely to cause a lot of content-based blocking over the next few weeks, until fixed rules are deployed both by SpamAssassin users and by all the various spam filtering appliances that use SpamAssassin rulesets.
(If you’re a SpamAssassin user, add “score FH_DATE_PAST_20XX 0.0” to your local.cf file to disable that rule).
EDIT: Mike has some more background on the bug.
EDIT: Fix it out on the spamassassin homepage.