I think the title sums up the question. I want to install Ubuntu via EFI, but I don't want to eliminate the OSX partition either (and I'd still like to be able to boot either).
Yes, as fossfreedom says, you can install Ubuntu in EFI mode on a Mac. One caveat is that hardware support seems variable (and is constantly changing). Some people swear that important hardware (like video devices) don't work properly in EFI mode. Others have no problems. As a general rule, support is improving, but I don't know if absolutely everything is supported yet. (OTOH, some hardware seems to work better in EFI mode than in BIOS mode.)
One more point: rEFIt is a boot manager. Although it can direct the boot process to a BIOS-mode installation, it won't disable an EFI-mode installation. In fact, it can be used as the first stage in an EFI-only boot process, enabling you to select Linux vs. OS X, both of which boot in EFI mode. (FWIW, rEFIt seems to have been abandoned. I've forked the project as rEFInd.)
Ubuntu 11.04 (Natty) supports booting from EFI as explained by this Ubuntu community wiki.
"From Ubuntu 11.04 onwards (x86_64 only), the ISO CD supports UEFI booting and the Ubuntu installer will try to setup the bootloader got (U)EFI boot."
However there is a warning:
The alternate CD is a text based installer. You will be able to repartition your disk to hold both your MacOS and Ubuntu.
As with all installations - make sure you have a full disk image backup first before repartitioning and installing just in-case the installation fails for whatever reason.