There are reports from WebmasterWorld that Yahoo Search is shuffling their search results. This is interesting, because we normally do not have a Yahoo Search update until the first week of the month.
The thread at WebmasterWorld is pretty convincing, in terms of this being a real update. We do not have a "weather report" from the Yahoo Search Blog yet confirming the update. But I am sure it will come soon enough.
There is also a DigitalPoint Forums thread discussing a spike in traffic from one webmaster. Hopefully, you saw a spike in your traffic as well!