BuddyPress.org

Try it on a test database, but if you upgrade to WP3.0. Then, you can just convert from single WP to multisite WP. I could be wrong, but I think that this should transfer all your data and preserve your old path (which is good for SEO, etc).

Like I said test it, but I’m interested to know if this works or not too. It will help me decide with this new BP install I’m doing if I should go with WPMU from the start or just WP.

Of course, this also would require you to move to WP 3.0 (which is currently in RC1).