BuddyPress.org

Folks,

The issue here with 3.0 is that if an existing WP 2.9.2 install with uploaded media upgrades to 3.0 and enables the network the blog option for the upload_path can’t be changed to the blogs.dir folder because the existing media becomes inaccessible.

For now, if you are looking to go to multiple blogs after install, then you should install MU 2.9.2 and upgrade that to 3.0 instead of installing WP 2..9.2.