I don't think that it matters. I have used shared hosting plans and have ranked well in the search engines.
The only reason I can see getting a dedicated host is if you are going to build some kind of mini-net where you have 10-15 sites all linking to each other and they all come from different IP addresses. I have seen some hosting companies advertising SEO plans, but they are very expensive and I think they are just making quick money off the whole SEO market and the myth that too many links from the same IP are bad.
I don't think IP addresses are a major factor in determining search engine rankings. It is more important to get quality backlinks over time.
