N/A as far as I understand simply means PR hasn't been figured yet. Where as 0 means, well you have 0 PR. I your PR is NA wait for the next update and you should get some kind of PR rating.
It's strange though how the N/A ranges in terms of the articles it's showing up on. I've got articles from 2 years ago showing N/A so it's not an issue with it being too new, granted I haven't built any backlinks to most of those articles. A really good example is articles I wrote maybe in December of last year and even created a handful of backlinks to so Google has no reason to not know they're there, yet they're listed N/A. Then I've got other articles written around the exact same time, same niche, same everything with the same backlinks and they're showing PR 1 or 2. There just doesn't seem to be any rhyme nor reason for why some of these are showing N/A. I can't see being penalized when I don't mess with the equation at all between so many articles.
While an unindexed page will report a PR N/A value, PR N/A is not an indication that the page is unindexed or penalized. PR N/A : at the time that you are checking the page rank there was no page rank reported. (i.e., PR = Null) Either, the page (not site) you are checking has been created since the last public PR update and it actually holds PR value but public PR records have not been updated to reflect the change. All pages, regardless of their actual PR value will hold a PR N/A value until the next public PR update. your page could actually be a PR 3 but still show PR N/A because public records have not been updated Or, there is are no other pages with enough PR linking to this page to pass sufficient PR to the page so that it can gain its own PR value. In this case the public rank has been recalculated since and still no page rank has been assigned. PR 0 : The page existed prior to the last public PR update. It does not mean that the page completely lacks page rank, it means that the page rank was not enough to reach PR 1 (e.g., PR = 0.5). between 0 - 1 still have value unlike n/a which has no value at all This is my take on it. I might not be 100% but i bet i am very close.