A thread at WebmasterWorld named Five Year Anniversary of Google on Yahoo!, where Yahoo picked Google to provide its default search results. I did some clicking and found the press release on Google's site; Yahoo! Selects Google as its Default Search Engine Provider. The press release was dated on June 26, 2000. I assume that the results switched over on this date, July 4th, 2000.
I hope most of you remember five years ago today (July 4th) when Google replaced Inktomi to serve secondary search results at Yahoo!. How many of us predicted then that Google would be as huge today, and paving the landscape of search? It is incredible what can happen in just five years.