BuddyPress 1.0.1 is now available. The easiest way to upgrade is through the plugin browser on your WordPress MU installation, you should see a notification for the latest version. If you would like to upgrade manually you can download the latest then simply overwrite your existing files. Please make sure you backup first.
This point release fixes a number of security issues in the 1.0 version so it is a critical upgrade for everyone currently running BuddyPress. There is also a number of other bug fixes so please review the release history for more information.