One of my sites is very new (about 3 months). When first launched its pages had (unsurprisingly) a Google PR of 'Unranked' [From Google toolbar stats, via the firefox SearchStatus plugin]. After a few weeks these changed to PR0. Just recently I noticed that they are showing PR 'Unranked' once more in Google Toolbar.
As far as I know I'm following the Google guidelines. Results for the site still seem to be showing for its keywords.
What could this mean?