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The reason why you should have the old host for another 1-2 months, is because Google have troubles with the DNS changes and often crawl the old site, until they refresh their DNS database.
I changed my site to a new server about 2 weeks ago, nearly every day since the only entries in the old server logs were FreshBot.... but a couple of hours ago it finally started hitting the new one. Think I've missed any deep crawl this month though. :(