We have been running DUC 1.2.2 clients after our server upgrade from Domino 6.5 to 7.0 and upgrading server side DUC from 1.2.2 to 1.2.3.
We haven't had any issues with 1.2.2 clients and 1.2.3 on the server. All of our clients are 6.5.x running 1.2.2 or 7.0.1 running 1.2.3.
Our Unity is 4.0(5) w/ ES45 (required for 4.0(5) and Domino 7 w/ DUC 1.2.3)