As far as I know, if you want to install BuddyPress you've got to have a WordPress MU installation (I know WordPress 3.x already is MU in itself) with a unique database, which has different tables for each blog.
But my situation is quite different: we have a blog network, and we would like to have a buddypress installed for all our blogs. This way users could register once and comment on the blogs automatically, see their activity, link to other users, etc. But the blogs are on different servers :(
Our current physical division of blogs (some of them are together in the same server, but others are in different servers, each with its own database) makes me think this is quite difficult to do. I know that the Multi-DB plugin could help us with the database thing, but... what if the blogs are, as in our situation, in different servers/VPS's?