Just thought I'd add a note about an issue I had installing multisite using wpmudev instructions:
As per emails I received from wpmudev on joining, I installed the dashboard first.
Proceeding to multisite, I could not get it to work until I deactivated the dashboard plugin.
Maybe add a line in the installation guide under "Updating your wp-config.php file" between point 2. and 4.
3. Deactivate all plugins
Maybe point 3. has been accidentally deleted (it is missing) and was about plugin deactivation.
Once I deactivated the dashboard plugin multisite worked.