root login fails mac osx 10.7.3

I am trying to use the Apple-approved method of changing a user account short name. This involves enabling the Root User, logging in as root, and then perform a few simple steps. I've done this many times before successfully on many different Macs. However on this particular MacBook Air, while I can enable root and enter the appropriate password via the "Other" choice at the 10.7 login screen, logging in as root fails. Here's what happens:

-- I enter username "root" and the appropriate password

-- it appears that the password is accepted

-- the Mac's screen turns blank as it usually does when one is ablout to enter a particular user account

-- Instead of the user desktop screen, the Mac simply returns to the login screen. There is no error message, etc. It simply fails to continue the login process to the root account.

I've tried changing the root password, resetting the PRAM, etc., and this behavior persists.

Any ideas?

thanks,

Eric