It's the whole bad neighborhood thing - it doesn't have any direct correlation to rankings, but it's not the best idea to have your website hosted on the same IP address as a bunch of (email) spammers.
That said, there are lots of good reasons to have a unique IP. The biggest one is SSL.
FYI: Yahoo has blocked entire class C blocks in the past, so do check out your neighborhood.
pretty much all the virtual hosts allow you to buy your own static ip for private SSL and any other purpose. my host is giving me one for 2 bucks a month soon... so whats wrong with virtual hosting if the virtual hosting company provides a static ip for your account?