Google knows far more than a simple index of your host. It also logs the originating host IP address and if you have several websites on the same host it aint so stupid as the count multiple links from the same C class IP address as full value...
Quite simply, Google knows who links to you, and they have excellent sorting algorithms which basically group all those links in multiple categories, they have an internal PR(or whatever) score which is calculated by relevancy, saturation, user data etc and these groups of links get an overall score which generally contributes to your websites search engine rankings position.
Or at least that's what they say. In fact, it's just an educated guess.... My personal belief is that if you own 2 sites of value, and interlink, then yes that is going to help with Page Rank, Trust Rank, Popularity and of course SERPs.
So really, in my opinion, whether you have 2 sites or a dozen sites, as long as they are run with good content and independently of each other (no automated scripts to generate content) then interlinking even from the same C class IP address is going to give your website/s increased authority.
Or of course you could set up 10/20/50/100/1000 websites from domains you have registered for one year each and cost you the hassle of setting the name servers, creating scripts to auto-generate content then meticulously place links from specific parts of each in the hope that despite no true value they will somehow contribute to your main sites popularity. OK - so yeah, that's just going to FAIL or work short term at the very best.
I don't think Google is immune to tricks but they follow the SEO marketplace carefully, as you would expect, and as quickly an idea like "lets reg 200 domains and point them all to our main domain" comes about they are pretty damn fast with addressing it.
If it's SE0 advice you are after

1.Article marketing. 2. 3rd party blog posts. 3. Relevant directories. 4. Relevant site links. 5. Forum profiles(with allowed anchor texts)