I experience a similar problem, and tried to start the upgrade from the terminal to see if any errors came up and here is what I get :
Not sure if this is important or symptomatic.
pierre@nubuntu:~$ gksu 'update-manager -d'
warning: could not initiate dbus
authenticate '/tmp/tmpcbxGmC/hardy.tar.gz' against '/tmp/tmpcbxGmC/hardy.tar.gz.gpg'
I booted on the live CD to check if the new version worked fine on that PC and it did. So I'm now frustrated.
Anybody knows how to resolve this ?