A WebmasterWorld thread discusses the procedure one should take when switching a site or server to a new IP address.
(1) Make sure all files are properly moved (2) Make sure the site functions properly (3) Try and keep your old site running on the old IP for two weeks after switching
The first two are no brainers. The 3rd, well, search engines run their own domain name servers (DNS), and those servers don't always update their records as quickly as other DNSs. So, that is why folks recommend keeping the old site on the old IP for at least two weeks. This way if the search engine's DNS does not update as quickly, it will still find a site to go to.
A week or so ago, we moved our physical server to a new data center. That means we did not have our sites running on the old IPs for any amount of time. What we did instead was lower the TTL (Time to Live) on the domain names to the lowest possible number and then migrated over. This is not the best solution, but for us, it was our only solution at the time. Ranking affects? Unknown... Traffic remains stable in my reports.
Forum discussion at WebmasterWorld.