I'm really impressed by Buddypress and plan to start working with it when WPMU 2.9 comes around (unless I get a reasonable timeframe for WPMU/WP integration with 3.0). I'm really interested in the new theme architecture, but even after listening to Andy on WP Weekly, I couldn't get a clear understanding of how the framework does/doesn't integrate with existing wordpress frameworks.

For example, Justin's Theme Hybrid seems to use creation of child functions.php and style.css as the major ways to customize Hybrid with child themes. I like this, but BP seems to take the direct override approach with new templates in the "child" theme directory (which is also possible in Hybrid, but seems less desirable).

Ultimately, I assume developers would want the WPMU, bbPress and BP theme frameworks to work through ONE child theme to create a unified look and feel for the entire site. Not quite sure if that's possible or even contemplated. Hopefully, someone will answer that question before WPMU comes out and I have to find out for myself.