BuddyPress.org

As JasonG states, WPMU is really setup to work with a single domain. However, as he also points out, Donncha has an early-stage plugin that allows the mapping of a domain to a specific blog on a Mu install. It is called the WordPress MU Domain Mapping plugin.

Concerning getting the multi-site plugin to work with BuddyPress, there is a ticket in Trac that addresses a potential way to allow it to work with BP.