The version of grub2 (1.99) in Ubuntu 12.04 has a bug that results in grub2 failing to boot on certain systems when they boot in UEFI mode. After investigating and working with the grub maintainers, we isolated it to a behavior of the BIOS in PowerEdge 12G systems, although one of our BIOS engineers also found that the UEFI reference implementation from Intel (TianoCore) exhibits the same behavior, albeit on a smaller scale. Patches now exist for both grub 1.99 and grub 2.00, the later of which will be available in Ubuntu 12.10. Unfortunately, the grub2 packages in Ubuntu 12.04 have not yet been patched to fix this bug, so installing Ubuntu 12.04 on a Dell system in UEFI mode requires some manual workarounds to install the system and then later install a grub 2.00 backport from Quantal.
We have asked for an updated grub2 to be included in Ubuntu 12.04, including the installer. Until 12.04 is updated, if you need to use Ubuntu 12.04 in UEFI mode on your 12G system and can use BIOS mode during installation, you can follow this summary of how to install from BIOS mode and later switch to UEFI mode.
Please note that this is only meant for advanced users and is not supported by Dell. It is also possible to modify the install media for Ubuntu to use a patched grub2 so that the media boots, but those steps are not included in this blog post. A follow-on post by my colleague Stuart Hayes addresses using modified boot media to install directly from UEFI mode.
To install in BIOS mode and then switch to UEFI mode, you can follow this summary of steps. I would like to reiterate that this procedure is only meant for advanced users.
deb http://ppa.launchpad.net/cjwatson/grub/ubuntu precise main
deb-src http://ppa.launchpad.net/cjwatson/grub/ubuntu precise main