From my experience, having your sites hosted on different IP's has plenty of an effect (positive)...but not for passing more authority than if they were hosted on the same IP...but so google doesn't ban ALL of the sites/devalue all of your sites.
What I mean is this:
About a year ago, I had about 25 sites all hosted with the same host. None of them were interlinking, and none were related to one another.
Some had a decent PR and even a decent natural SERP position for medium competition keywords.
Then I thought "hey, since most of these sites have at least a PR 1 why not link them ALL together to boost the PR of all my sites and hopefully get an even better natural SERP position"
Can you guess what happened? All my sites plummeted in the SERP's.
When I say plummeted, I mean it. One of my sites ended up somewhere near page 65.
I'm pretty sure having your sites hosted on different IP's isn't so much so they pass more authority on to one another, but so that google doesn't know that they are all hosted by one person who is trying to make a "link farm".
I can't imagine what happened to me happening to you if you're only going to do it with 5 sites or so. But start doing it with 20 or more, and you're setting off a pretty big red flag.
I set up "link farms" all the time now, and VERY successfully (sometimes with astonishingly great results), but I'm spreading my sites out on 19 different hosts.
