I don't think this merger has anything to do with 2.8 taking so long. You can blame WordPress 2.7 for that. I'm not sure what code has been merged over from one into the other just yet. Hopefully we find out these details in the dev blog post.

However, I think back to when Andy Peatling said BuddyPress would be available for stand alone WordPress but at a much later date. So I wonder, if he knew way ahead of time that WPMU and WP would be merging together and thus, that is how BuddyPress would end up running on the stand alone version of WordPress. Hmmm