WPMU follows Wordpress Development. Currently, only Donncha O'Caoimh has commit rights. Check-ins can been seen on the WPMU SVN Changeset Log
This has the advantage that one person is held accountable for getting changes into the code base. It also has the disadvantage that one person is held accountable for getting changes into the code base!
A lot of code is submitted and discussed in the WPMU Forums. The forums do not allow editing of other's posting entries, so contributions tend to pile up as subsequent postings, and there is little or no deletion of obsolete content.