BuddyPress was formerly only available for WPMU (multi-user version of Wordpress). They just announced a version that works with the normal version of Wordpress. See the post here.
I tried loading BuddyPress on my Atahualpa themed site and it totally broke the page menus. It seems that BuddyPress menus appear exactly where the Atahualpa page menus appear. So my question for you all is "Has anyone figured out how to make BuddyPress work with Atahualpa?
UPDATE: I did figure out how to get the Buddypress menus to co-exist with the Atahualpa page menu. Here is what I did: In the Atahualpa theme options page, go to the "Style and Edit Header" menu option, then under "Configure Header Area" make sure the menus come after the logo, etc. So you will replace the default (which is "%pages %logo %bar1 %image %bar2") with this: "%logo %pages %image". (Leave out the quotes.) You can also use the %bar1 and %bar2 if you want to. But have the logo first.
Next, go down to "Logo Area: Styling", and add this: "margin-top: 20px" That is going to add a bit of space above the logo, sufficient to contain the Buddypress menu.
That's it! I ended up not using Buddypress for other reasons, but with those changes, the Buddypress menus do seem to fit OK, and can peacefully co-exist with the Atahualpa menus.
Three cheers for Atahualpa, a totally awesome theme!