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I think that now a days, with many sites using reverse proxies (such as CloudFlare) Google has stopped looking at the IPs as it is no longer a factor to rank sites. Jeff's point on the e-mail problem is right, however, you can simply host your email on a third party IP, like Google Apps, and never get blocked. Something that even dedicated IPs get the punch. I have a few dedicated servers, some of them with multiple IP addresses and I found myself trying to clean new IP addresses that just were to new or "abused" by previous owners. Switching hosting provider, however, while changing the IP address may increase/decrease rankings not because of the IP but most likely because of the site speed, which IS something that Google considers to rank sites.