Cluster-Aware Updating is a neat little feature added by Microsoft - it easens the burden of system administration by giving the administrator some extra-comfort. Namely, it can speed maintenance up by quite a bit by automatically applying a sequence of system updates to every node in a row (i.e. a cluster).
This feature is also quite easy to add to your existing cluster - just pick the Cluster-Aware Updating link sitting inside your Failover Cluster Manager and follow the wizard.
However, should you encounter any problems with the creation of the AD account, try pre-staging the CAU AD account so that users other than Domain Admins can add the account ("Everyone" should work):
That should do it - your cluster should be ready for self-updating procedure...