I recently began developing in Wordpress MU for big project at work.  I found out real quick that there are not nearly the free resources available out there, nor the forum helps as regular wordpress installs.  It didn’t take too long before I stumbled across WMPUDev.org.  They have a ton of AMAZING plugins that got me up and going really quick.  Check them out if you are looking to do any development with Wordpress MU.