based upon user feedback on this subject on other boards it almost seems as though the major search engines treat each profile as a new domain entirely. even though someone's profile may read PR 8 or something, there's a 99.9% chance you will NOT inherit it.

not to mention myspace profile suffers instant duplicate content.
whereas: http;//myspace.com/mytoken=blablabla (you know, the long url's?)
and:
http;//myspace.com/customusername are the same exact page

unless you're just looking to meet / see your visitor's faces, you'll probably benefit more greatly by SEOing your site the old fashioned way.