A PageRank of ZERO (0 page rank) means that the page has a pr above 0, but still less than 1. Keeping in mind that, Toolbar PageRank display is limited to only whole numbers ranging from 0 to 10, but Google's internal PageRank data has much finer levels of gradation.
When the Toolbar PageRank display shows a gray bar (n/a rank), Data Not Available, it has traditionally meant that either Google has not indexed the page yet or the site has been banned by Google.