I can't update 12.04 LTS. The Update Manager shows this:
CD/DVD 'Ubuntu 12.04 LTS _Precise Pangolin_ - Release i386 (20120423)' is required Please insert the above CD/DVD into the drive '/media/cdrom/' to install software packages from it.
The image I downloaded and burned to CD is from http://releases.ubuntu.com/12.04/
the PC (Intel x86) desktop CD version. I burned many CDs but the result is the same.