In 2003 Craig Silverstein was clear that well configured virtual hosts (Apart from 3% which are wrongly configured) will have their site treated in exactly the same way as a dedicated IP. A well known New York SEO consultant told me recently though that IPs with multiple sites will be treated as less important because they look less like major sites. This makes sense. While a virtually hosted site code be significant in the long tail it is less likely to be a major site.
More recently (November 2006) Matt Cutts also indicates that there is nothing to worry about. We are talking here about multiple names being on the same physical IP. Something which is easily achieved under Appache with a ServerAlias. If you lookup your domain with nslookup or by looking at the IP address revealed by a whois service and then entering that IP address as a URL. If the URL results in your web site then you probably have a dedicated IP and are at least the default host for the IP. If you don’t see your web site then this is because the hosting provider is using name based virtual hosting which presents the appropriate page depending on the URL used.
One physical machine can have multiple IPs, normally through the use of multiple physical network ports and the search engine is unlikely to be aware of this sharing of resources. If however multiple domains are mapping to the same IP the search engine could in theory take note of this and deduce that your site doesn’t require the bandwidth of a dedicated network connection and therefore can’t be so important. This is likely to be correct logic and significant other than for a long tail site.
There are other hosting arrangements where an individual domain maps to multiple IPs which operate across multiple servers in order to provide greater resilience. An omniscient search engine might draw the conclusion that the bandwidth of multiple network connections shared across the number of domains sharing them is providing an indicated total amount of bandwidth. So far there seems to be little evidence that the search engines do such a calculation.
So though it seems reasonable that for a site wishing to appear very large (And we could argue that is all of us) their should be an advantage to using dedicated IPs, in practice nobody is displaying any evidence that there actually is an advantage.
