From Searchblog on Google A/B testing:

The ideal number of results on the first page was an area where self-reported user interests were at odds with their ultimate desires. Though they did want more results, they weren’t willing to pay the price for the trade, the extra time in receiving and reviewing the data. In experiments, each run for about 8 weeks, results pages with 30 (rather than 10) results lowered search traffic (and proportionally ad revenues) by 20 percent.

This is actually a major point of contention for designing webpages & managing conversion . . . the # of results per page . I would caution against applying this globally to all websites though. Users tend to get anchored to thier prior experience with that particular website, so the drop in conversion could simply be caused by the fact that its a different experience than before. In fact, I’ve seen opposite results before. Anyways, data like these should be shared more globally, kudos to Google for being open.