Robert_Charlton - 10:05 pm on Apr 13, 2011 (gmt 0) [edited by: Robert_Charlton at 10:33 pm (utc) on Apr 13, 2011]
If you interlink your domains to boost ranking, then hosting separation is a consideration. That's not the same as one IP per domain... it's IP separation for sites that interlink.
It's been pretty widely discussed, but not recently. Here's a Google patent filed 2001, granted 2003, which discusses "local score"... which we've generally called "local rank"....
Ranking search results by reranking the results based on local inter-connectivity [patft.uspto.gov]
(PS: I don't think it's a good idea to interlink commonly-owned domains to boost rankings, but that's another discussion.)
[edited by: Robert_Charlton at 10:33 pm (utc) on Apr 13, 2011]