The first place to start is the readme file. Read through that entirely before posting to the forum if you have a problem.
The Codex is a mine of information about WordPress and WordPress MU. The WPMU index page lists all the related categories in an easy to read list. The list of recent changes is also worth keeping an eye on if you're interested in a particular area.
Trac is our bug tracker. If you think you have found a bug, post a description to the forum where you may be asked to open a Trac ticket. Trac is also a good place to search in case your bug has been fixed. The complete source code for the project can also be browsed here.
The WPMU chatroom on IRC is #wpmu on Freenode. Here are the connection instructions you'll need if you're not familiar with IRC.
