It's also why you can install Linux. If you can get Linux to boot in pure EFI mode (i.e. without BIOS emulation), I'm interested to hear how.
I went deep into this awhile back. Unfortunately, when Apple decided to go with EFI for its Intel Macs, the current EFI specification didn't meet all of their needs, so they added a few features, and standardized on that. Most non-Apple EFI hardware now uses the later EFI standard, which has those features, but in a way that is incompatible with Mac OS. The specific features escape me for the moment.
There's also a question of 32-bit EFI vs. 64-bit EFI, which finally made me back down and give up. I had applied kernel patches, gotten an EFI version of GRUB, started digging deep in the EFI shell provided by rEFIt (bootloader for Macs), and I couldn't get anywhere. I gave up.
