BuddyPress.org

You don’t need the BP Template Pack if you’re doing this.

What you’ll need to do is:

1) Deactivate the BP Template Pack plugin.
2) Activate the BuddyPress Default theme on your blog.
3) Setup WP 3.0 in network mode as a subdirectory install.
4) Create a new user blog and activate whatever theme you want on this new blog.