Multiple sites on the same IP have no bearing what so ever on SE rankings and or placment. Name based virtual hosting is something that is not only encouraged with regard to IP space limitations, it is required by ARIN. That said, there are going to be tons of sites that are hosted by a particular company, possibly on either the same IP or Netblock, all of which may or may not be owned by the same person and or persons. That said, penalizing a site based on its IP address would generate far too many false positives IMO.
Well it's all theory, but as Mia just also stated it would screw up the SERPS dramatically because of shared hosting. I really don't see how google could implement such a tactic and stay in business. That said apparently they do have the technology to do so if they wished.