Mod Rewriting Subdomains to Directories

[stevehnsn]

Say I had an established social community, where each user has his or her own subdomain (no, I did not design it), eg. username.website.com. These subdomains are created when the user registeres. Now say I wanted to SEO that thing by submitting user provided blog posts, videos, and music, to social sites and directories. My idea, to focus the attention on the root site, would be to rewrite all those subdirectories as follows: www.website.com/username.

What do you guys think are the negative impacts of mod rewriting this, leaving the underlying problem of subdomains alone?

Thanks.