Same problem for me; drastic decrease in indexed pages, as specified by "Site:" command. # is decreasing by the day.....seems as if there is no end to the downward spiral......However, the "site:" # does not correlate at all anymore to the WMT #, which has decreased some, but appears more stable.
Old news, this has been going on since September 2009, in WMT usually you can see low crawling volume for your sites.
The URLs where there is less crawling/indexing drop from the index if a URL has not been reindexed within 2-3 months. If this is the case, the cache date disappears next to the URL first and then the whole URL will be removed.
We have also seen a dip in indexed pages numbers both in WMT and through site: command about a month ago. Now they are more stable however, WMT still shows 50% less pages indexed from about a month. The contradicting point is that the number of top keyword occurrences show an improvement in WMT which IMO should be directly proportional to the site indexing. So I am assuming that the site indexing stats in WMT is just a glitch while at back-end Google is indexing pages