Hmmm... could it be something you installed recently? Usually things don't break down it Mac OS X without doing something first. :)
Try disconnecting and reconnecting the webcam. Also, can you see video of yourself? (I'm not sure if yahoo messenger has this feature.)
If it still doesn't work, try deleteing the yahoo preferences file. It's located in your home folder/Library/Preferences/Yahoo Messenger Preferences.
John A
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