well , the one solution I use is to use mod_rewrite to convert all links to canonical form. This way, user or spider requesting site.com would get 302
www.site.com and PR will get transferred. I am not sure at which stage PR is assigned, but after using this for a few years on a site with many inbound links created by users which I never asked for links (so they are coming in all forms) I just checked and I got PR0 on domain.com and PR6 on
www.domain.com, so I guess it works fine. Oh, and absolute links or a <base href= is a must, too.
If you need the code, let me know. It's trivial really.