Re: Help with Upgrading my Site
If you have bbPress installed, you should double-check to see if your BP group forums work.
Post in bbPress in a BP group forum and see if that post appears in BuddyPress and vice versa.
Also if you used any BP widgets, remove and re-add them since the widgets API in WPMU 2.8 changed.
Lastly, check to see if your 3rd-party BP plugins work (if you have any installed).