I'm not sure, but I think it was supposed to include the Unity desktop during the upgrade process too. I did a clean install of Ubuntu 11.04 so I can't say for certain.
Although, one possible reason for Unity not getting installed can be because prior to upgrading maybe you didn't have a package installed that requires Unity as a dependency, such as 'ubuntu-desktop'. Check to see if you have ubuntu-desktop installed. If it is not installed, installing 'ubuntu-desktop' should correct the problem.
A reverse dependency check for Unity on Ubuntu 11.04 says:
Those would be all of the packages that require Unity.
james@Ubuntu:~$ apt-cache rdepends unity
Hope this helps.