The scale is believed to be logarithmic. The early Google papers mention that the scale was logarithmic back then, and every quantitative analysis (reverse engineering) I'm aware of comes to that conclusion.
In a log scale, PR5/PR4 = PR4/PR3 = PR3/PR2 etc., a very useful feature (notable Pareto/Zipf Web analogies have been made by Nielsen and others).
People disagree over the 'log base' (assuming a coefficient of 1); in other words "A PR6 is as good as how many PR5s?"
Nothing is quantifiable about Google these days, IMHO. I have 37 backlinks according to AlltheWeb, including a PR6 and two PR4s, many of my pages are indexed, I'm not banned, and all of my pages show PR0. It's not pretty.
I don't see your site in your profile, so I can't check for you, but do be aware of the generally-consistent screwiness of the toolbar's showing of PR.
All but one of my pages show "Current page is not ranked by Google" (PR0), but at least as of a week ago, several kind WW'ers informed me that my pages mostly showed up ranked as a PR5 on their toolbar.