I'm by no means an SEO expert, but it seems pretty clear that a unique IP cannot count too highly. I say this because many (if not most) hosting services utilize virtual domains on common servers. For example, the server of my domain hosts 256 other ones, all unrelated to me:
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http://whois.sc/r0k.us
It would not make sense for search engines to penalize sites just because they happen to be served by the same hosting company.