If being your own brand is so important than why not just get your own domain name. Also, these profiles on about.me seem to just provide short bio with some links to twitter, flickr, facebook, linked, etc... You could do just do this directly on your tumblr, posterous, linkedin etc.. page.
Someone posted a link today to the PG essay on stuff. The site about.me makes me realize that maybe sometimes people collect too much virtual stuff. About.me seems to be just more social clutter in the tangled spider-web of a user's online profile. I don't see how about.me adds more value.
Linkedin comes the closest, but it's purely professional, pointing someone to your linkedin will only give them a limited view of yourself.
Being your own brand is an important socially as it is professionally. So it makes sense that people are going after this market.