"You are unable to log in to the user account [account name] at this time.

Problem:  an AD bound Mac shakes off login attempts and returns a message that says:

"You are unable to log in to the user account [account name] at this time.  Logging in to the account failed because an error occurred."

There are two things to to try:

First, update the Automounter master map as outlined in this Apple KB article:

http://support.apple.com/kb/TS3346

Secondly, if the user has a home folder path specified in their AD profile (Profile tab), remove it.