From my experience, when Google de-indexes a site, they also suppress any PageRank the Google Toolbar would have shown for it; that doesn't appear to be the case here, digg.com is still PR8.
Having toolbar PageRank[1] and yet no cached page[2] is not something I've seen before.
[1] http://toolbarqueries.google.com/tbr?features=Rank&sourc...