Confirmed, same problem here, all updates 28th Feb 2009, this is probably because its dependencies haven't been rebuilt with python 2.6, you can run the command it says without breaking Ubuntu, and just wait until they are rebuilt, then reinstall ubuntu-desktop in the usual way (sudo apt-get install ubuntu-desktop)
Confirmed, same problem here, all updates 28th Feb 2009, this is probably because its dependencies haven't been rebuilt with python 2.6, you can run the command it says without breaking Ubuntu, and just wait until they are rebuilt, then reinstall ubuntu-desktop in the usual way (sudo apt-get install ubuntu-desktop)