If, for example, your site is a PR4 without the "www." but a PR0 with the "www.", and you want to pass the PR accross, all you need to do is link your title or logo right at the top of the homepage to "http://www.yoursite.com", so that whenever Google visits "http://yoursite.com", the first link it comes accross is the link to "http://www.yoursite.com". Then, by the next PR update, it should have passed at least some of the PR across.
I had a site that was PR0 with the "www." and PR5 without. Now after using this method it is PR4 both with and without.