It is really odd. I have some old blogs (5y+) which have a decent PR and perform extremely bad, while some newer ones (under 1 year old) that are still N/A and perform excellent.
Best suggestion is probably as always to not care about PR but just focus on the actual performance.
Mat Cutts had said that page rank was going to be updated 'in a couple of weeks', a couple of weeks ago. Probably it's being updated right now?
Notticed the same thing today on all of my websites, glad to hear I am not the only one!