leadegroot - 12:18 pm on Jun 20, 2010 (gmt 0)
Has the bot visit frequency changed?
Have your rankings changed?
If you aren't able to assess the raw logs and the site is verified on webmaster tools, I would click through all the pages looking for something odd - it reports bot visits, so you could look at that.
It is possible (but extremely unlikely) that you have a case of poisoned IPs. (An issue with the IPs on the new server that means the bots have them prelisted as banned, thus they don't visit, thuse your rankings fall through the floor).