The lowest absolute pagerank a page can have is 0.15 due to the “(1 - d)” part of the equation. But this is likely to show up as 0 on the Google browser toolbar tool.
Your site is likely to be de-indexed if it has no links inbound though - I do remember reading that the Pagerank engine does do an optimisation before it calculates the PRs in that it removes pages from the database that have no links to them.
This is quite reasonable given the original motivation of pagerank:
PageRank can be thought of as a model of user behavior. We assume there is a “random surfer” who is given a web page at random and keeps clicking on links, never hitting “back” but eventually gets bored and starts on another random page.
If a site or page has no inbound links then the “random surfer” can never get there. (well, ok, unless the “random surfer” starts out inside your site of course…)
But it’s quite hard to have absolutely no inbound links. It only takes just one public link for your site to be “on” the web. DMOZ is a good place to put your one link if there’s absolutely no other avenue open to you (though this is very unlikely)
