Yahoo! Search is currently undergoing an update. If you noticed your rankings shuffle around, it is true. A WebmasterWorld thread first spotted these changes yesterday morning.
Tim Mayer of Yahoo! said;
There will be more changes tomorrow. Things should settle on Wednesday Tim
So a relatively quick update. Keep an eye out for what was changed.
Oh, let me note, if Google did an update now, they would be nailed up against a wall for messing around during the biggest shopping season. Hence the denial of the update, which some say happened and Google says did not.