Comment 9 for bug 907390

As I see it, this is not a bug in the kernel but instead:
1) A decision taken by the Ubuntu TB
2) A change to be requested in Virtualbox

Daniel provides a working solution on how to get 12.04 to run in VirtualBox, see comment #5.
I have assigned this bug to virtualbox. I recommend that virtualbox turn PAE/NX on when creating Ubuntu VMs from now on.

James makes a good point as well. A workaround must be in place to install 12.04 on non-PAE enabled hardware. We used to install PAE only when 4GB or more of RAM was detected and that the system was online when installing. I have been told adding the non-PAE kernel would add ~40MB, therefore it is not possible to have both kernels available on the CD. To install onto older (non-64-bit) hardware, we would need a separate CD for older systems or to revert to what we have been doing up to now.

You can read more on the decision from the Ubuntu Technical Board here:

http://www.phoronix.com/scan.php?page=news_item&px=MTAyNzM

Note the following quote: "Switch precise over to PAE kernel by default on i386; we retain the option to revert if it causes too much fallout (Colin)"