WordPress MU

I set-up and had running a WPMU install on an IP address.
All was working fine, back and front end.
I then changed nameservers and tried to access via the domain name - front end works fine but the back-end will not let me login.
I access domain.com/wp-admin enter the details, and the page refreshes, the URL changes to the correct redirect= URL but that's it.

I simply cannot log into the admin
The site is at root level - domain.com
I can access phpMyAdmin and ftp with no problems.
Any help would be appreciated.

The domain is stored in the db in a multitude of places. Go in phpmyadmin, search for the IP address and change them all to the new doamin name.