Google counts links from the same sites and same IPS, but it is believed they count it a little less than they do for other sites/IPs.
As far as PageRank goes (Google's Version of Link Pop) It doesn't matter. You get the PageRank no matter which site/IP you are on.
Other sites, such as Altavista have been known to ban entire C Blocks and do a much more arbitrary job of link pop. For those engines it is better to be on different IPs.
I have even heard, and I believe it has some truth to it, that AltaVista goes through the zone files to match things up based on who a domain belongs to.
If you really want to be safe - you would need:
1) Different DNS Servers.
2) Different C Blocks
3) Different Owners listed under contact info.
Note, I do not think this is very necessary, but there have been papers written on 1 & 2, and it is not hard to believe Google might start doing this (to some extent) in the future.
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