I agree, on my first site with no knowledge of PR i got a PR5, but then it went down to PR4 since I didn't promote it, but then the forum got PR3 and so did inner pages.
Blog commenting works if you post thoughtful comments that make it obvious that you read the blog post. Try to say more than just two sentences also. Don't use a key word for your name either. Try to post on blogs that are related to your site. Important* Use Firefox and install an SEO plugin such as SEOQuake and/or Search Status. They will show you if the comments are nofollow. If so, you won't get any juice from the comment you placed. You may get traffic though, which is always good.
I made a depression website with 100 articles and used the word keyword throughout the articles with a frequency of 3%. I did not submit to many directories or anything but according to the google toolbar I had a PR of 4 within a month. So I think it has more to do with with content and the frequency of keyword and you also need to keep updating because my website now has a PR of 2 since I have updated it.
the point is to get more high pr backlinks, but get free high pr backlink is not easy, except your website is really nice and useful
My design site got PR3 on fist update and it only had one link coming from my PR3 arcade site. It stayed PR3 for a year until I canceled the registration renewal. I think it helped that my site was valid XHTML on every page.