Forum Moderators: open
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.
Google's denied punishing virtual hosts in anyway but they have a patent application which states how they would examine your nameserver.
In believe that it's more a matter of choosing a good host than of choosing a good IP address.