comScore released their January 2009 report on search market share, and for the first time in a while, Google's share declined month to month.
In December 2008, Google had a 63.5 % share, but in January, they dropped down to a 63.0% share. Yahoo gained month to month with a gain from 20.5% to 21.0%.
Google Sites led the U.S. core search market in December with 63.0 percent of the searches conducted, followed by Yahoo! Sites (21.0 percent), Microsoft Sites (8.5 percent), AOL LLC (3.9 percent) and Ask Network (3.7 percent).
In any event, you need to understand this is month to month and you need to look at the numbers over time. So I wouldn't jump to any conclusions after looking at the monthly numbers.
A WebmasterWorld thread is discussing this and one member said something very funny, as to a possible explanation:
maybe it had to do with that malware issue to which google was linking people
Could be... They go to Google, do a search on that Saturday and freak out and leave. Then they head over to #2, Yahoo.
Forum discussion at WebmasterWorld.