We often have customers ask having if a unique IP address is required for good Page Ranking with the Google search engine.
This information is from a July 2002 Slashdot interview with Google Director of Technology Craig Silverstein:
Q: Why in this day and age does google continue to penalize sites that are virtual hosted? With ip addresses becoming harder to get/justify every day why does google discount the relevance of links that don't come from a unique ip address?
A: Actually, Google handles virtually hosted domains and their links just the same as domains on unique IP addresses. If your ISP does virtual hosting correctly, you'll never see a difference between the two cases. We do see a small percentage of ISPs every month that misconfigure their virtual hosting, which might account for this persistent misperception--thanks for giving me the chance to dispel a myth!
Full text: http://interviews.slashdot.org/interviews/02/07/03/1352239.shtml
And a 2006 follow-up posting by Google employee Matt Cutts: