Brett Tabke has split out a thread that has been getting a lot of attention named Pages dropping out of the index. GoogleGuy replied to that thread stating;
One thing to bear in mind is that Bigdaddy will have different crawl priorities. That can account for some of it. If you've run into any spam problems in the past, you might also want to do a reinclusion request. Otherwise, please send an email to bostonpubcon2006 at gmail.com with the subject line "crawlpages" (all one word), and I'll ask someone to see if they notice any commonalities.
This post, deserves its own thread, so Brett split it out and named it With Googles New Big Daddy Index, Crawling is Changing .
Currently we don't have much more information, than what GoogleGuy posted above. Does "different crawl priorities" relate to Google's new Crawl Caching Proxy? Or is it cache independent? I believe it has to be cache independent, due to the volume of posts in the original thread.
Forum discussion on GoogleGuy's comments at WebmasterWorld.