This happened to 1 of my slow death site too. Trawler,
It may mean that
1) you do not have any redirect on non -www version (site.com)
2) From some referal Link, Googlebot crawled non -www and indexed it like site.com/x.htm, site.com/y.htm
3) and this may have shooted the Mirrored Content Penalty, by comparing www and non -www version.
I still have 2 pages in index without -www and with tag of supplemental results.
Yeah Google is 100% having problem with dns cache
This happened to 1 of my slow death site too.