The software behind Akismet underwent an upgrade tonight, which you should only notice if spam was getting through the filter before. As always, vote with your submit buttons. If you notice any mistakes mark the comment as spam or ham (not spam) and it will be submitted back to Akismet, which will learn and adapt.
This was the first major upgrade since Akismet launched. I also took the opportunity to upgrade some of the server software to the latest versions, which seems to have performance ever-so-slightly better.