Off the back of a recent SCCM implementation I did at a school, I had an email yesterday from the customer querying why the domain join step in their newly created task sequence was failing, even though the credentials entered were correct and all other steps were working perfectly. I remotely connected into their server to find the following:-

The problem here is that you need to specify an OU when using the ‘Join Domain’ step! If you don’t select an OU here then SCCM will fail even though the wizard doesn’t identify it as a mandatory field. Seems pretty obvious, but if the wizard lets you OK it then it should default down to the Computer container right? Wrong. Hope this helps.