According to BuddyPress, one can use any theme without modifications. However, it doesn't quite work right. I would love to see Hybrid as the engine/main theme behind a site that uses BuddyPress and WPMU.
I've tried to do it myself, but I just don't know very much. I see that you have to add buddypress-home theme files into hybrid theme files to be able to use it as the home theme of a buddypress site (plugin-sidebar.php and plugin-template.php) but then it seems like hybrid goes to no-sidebar. I also moved over some of the functions.php. And there are 404 errors. So it is not quite seamless and I'm not knowledgeable enough to do it myself.
BuddyPress just went beta. I would think more and more people are/will be on the market for another home theme from which to build a social networking site. Hybrid would be an awesome theme engine to run a social network site, it just needs a couple more things (I think).
If you were to consider putting out a Hybrid that is BuddyPress ready, I would like to see it pretty simple in style, so that one could skin it easily. I do allright with skins, just not with code.
I am initially addressing this to Justin, but also to ask if anyone else would be interested in this. If there isn't interest, I will consider posting my request in the Jobs area to see about paying to have it done just for me.