According to Ubuntu Engineering Foundations team manager Steve Langasek:
Upgrades between LTS releases are not enabled by default until the first point release, 12.04.1, scheduled for July. It is recommended that most LTS users wait until then before upgrading to 12.04.
If you choose to upgrade before then, you can pass the
-d option to the upgrade tool, running
do-release-upgrade -d or
update-manager -d, to upgrade from vanilla 10.04 to 12.04. (-d stand for devel)
This is what the release notes say:
Upgrades from Ubuntu 10.04 LTS to 12.04 LTS do not work using the alternate CD or the server CD as a package repository. It is recommended that users running Ubuntu 10.04 LTS wait for the 12.04.1 LTS point release, scheduled for July, before upgrading. (988941)