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For example, one of my clients has a large block of IP Addresses and they shuffle things around for various reasons from time to time. There is literally no effect on their rankings from this.
The only dip we saw was because we also changed to a new CMS during the move and every single page on the site had a new filename. A combination of 301's, xml sitemaps, webmaster tool diagnostics, and time got us to better rankings than ever before.
I had a couple of forums, so after moving and changing DNS I 302 redirected from the www on the old server to www2 on the new one. Really dangerous stuff there. But in my view the correct way of doing things for users that can write to the server (saves them writing to the wrong server).
Traffic is up so far - nasty routing trouble with old server gone away completely. I'll post back if anything nasty happens.
We did leave the old site active for about 48 hours after moving just to make sure our site was found by routers that had our IP address cached and weren't scheduled to do a DNS lookup.