Just tried to register on your site and I get a white screen after hitting submit.
Please deactivate any plugins other than BuddyPress (if you haven’t done so already).
Are you able to access your server’s error log? If so, there is most likely a message in there. It would help if you could paste the contents of this log here.
Also you say you upgraded to WordPress 3.0. Do you experience the login issue you noted in point #3 with BuddyPress deactivated?