In a thread over at SEO Chat named Am I dreaming or yahoo shows google serps again? a forum member discusses his encounter with Google results in the Yahoo! Search search engine results page. Just like old times?
The conclusion was that it was exactly like "old times" thanks to some old cookies. What the people had to do, to stop this from occurring, was to delete their cookies, specifically the "google cookie" from Yahoo.
That got time to come back to 2004 for Yahoo! Search.