So some guys decided to create a PERL script which generated 10,012 different searches at Google and Yahoo with randomly selected keywords from an English worldlist. The test took them 18 hours and based on their results they conclude Google's index is actually on average 166.9 percent bigger than Yahoo's index.
In fact, in the 10,012 test cases we ran, only in 3% of the cases (307) did Yahoo! return more results. In 96.6% of the cases (9676) Google returned more results. In less than 1% of the cases (29) both search engines returned the same number of results.Seems like there is something fishy with Yahoo's claim of having 19.2 billion documents in their index while Google only has 8.2 billion. Read on over here for the full report.
UPDATE (July 20th, 2011): Link dead, data now available at Aprendelo.