Canonical is supporting an upgrade path for non-PAE hardware. From the release
notes:

https://wiki.ubuntu.com/PrecisePangolin/ReleaseNotes/UbuntuDesktop
https://wiki.ubuntu.com/PrecisePangolin/ReleaseNotes/UbuntuDesktop>

"The Ubuntu 12.04 installation image does not include support for old computers
that do not support PAE. If your computer is affected, you can either first
install Ubuntu 10.04 or 11.10 and upgrade to 12.04 or you can use the Lubuntu or
Xubuntu images. The non-PAE version of the Linux kernel will be dropped
completely following the 12.04 release."

I'm currently dual-booting Ubuntu 11.10 (Unity 2D) and OpenSUSE 12.1
(KDE-Plasma, with desktop effects) on my "obsolete" Centrino - Pentium M laptop,
so I'm a candidate for this upgrade path. Maybe I'll try it as 11.10 approaches
end-of-life.

Funny thing: I've got a still-older Pentium 3 laptop that doesn't seem to have
this problem.