Just had a read here http://www.theadminzone.com/forums/showthread.php?t=19498 It does seem like quite a few VB forums including mine lost a lot of cached pages. Going from 30k to 700 in my case. Do you think archive might be to blame?
Google admits that there are big problems with their SE that they are in the process of fixing. Googleguy at webmasterworld replied to a very big thread that he was looking into the problem, and then that he was getting it fixed. See my thread for a tool that lets you check the number of pages that you have on your site - google site: command against multiple datacenters. I also have some links there to the webmasterworld quotes. Google cache checker