There has been debate for awhile regarding whether or not using a static IP address is better than having a dynamic IP address for your website. By preference myself and I think I speak for many SEO's, we prefer the use of a static IP address when hosting a domain. Most often you can pick up a dedicated IP address from most hosts for a couple extra bucks a month. There are many reasons for this, such as for linking, avoiding penalities on the same IP C Classes blocks, and so on. If you are doing a major site, its just a better idea to have a dedicated IP than a shared one.
I just had lunch with Ian McAnerin which I owe this credit of information to as him as he spent some time talking with Google Engineers last night at the Google Dance.
Generally speaking having a website on a shared IP address will not cause you any harm. You can rank just fine and there is nothing negative about hosting on shared IP's. So don't worry if you site is on it. However what is important to know as I was told is how, Google in particular and possibly other engines look at these static and shared IP. As I was explained when I search engine spider first comes across your site it will parse it with basic HTML 1.0. If its able to do this then it will normally go about getting spidered and indexed. For those sites with a static IP address they will get spidered with HTML 1.0 as its able to resolve the address immediately. If by chance you don't have a static IP address, Google may go about parse the site with HTML 2,3,4 and so on until its able to resolve the address for your specific site. This can take up to 3 months to happen. In the meantime it will use the IP of the main site on this IP, often times the hosts site. It will come back until its able to find your specific IP. During this period of 1-3 months, any links that you build to your site that is found by Google, will get credited NOT to your site, but to the main root site on the shared IP, often times your host site. Quite interesting. As I understand it, after Google is able to correctly identify your site from others, you will get credited for the links. However in the time period of limbo between that, another site will be getting credit for your links. Something which you probably don't want to happen. Interesting.