I did an upgrade on my local install to WP 3.0 (with multi-site) and BP 1.2 beta. The upgrade went smoothly and everything seems to be in place thus far.
The one thing I have noticed is that any of the buddypress themes from here at wpmu premium dev give me the error message:
BuddyPress is ready. You'll need to activate a BuddyPress compatible theme to take advantage of all of the features. We've bundled a default theme, but you can always install some other compatible themes.
The themes actually do show up correctly when activated and the message is not there when I use the default buddypress theme. I've tried the business, magazine, and social buddypress themes, and they all give me the same message in the admin.