For those who don't know, EFI is a new boot environment for PCs that is intended to replace the old BIOS environment that PCs have used in various forms since the first IBM PC shipped in 1981. EFI is much more advanced than BIOS, but only the most recent operating systems can boot on EFI-equipped PCs.
So why is this interesting? Apple Intel-based Macintosh computers (which is everything sold since mid-2006) use EFI and Mac OS X requires EFI in order to boot. (There have been some hacking efforts to make Mac OS X run on BIOS-based PCs, but they are of questionable reliability.) With the EFI-X Boot Module, you can get that same EFI software on a generic PC. It would appear that this alone is sufficient to allow Mac OS X to install on non-Apple hardware. (Assuming you have hardware for which Apple provides drivers, of course.)
Of course, installing Mac OS X on non-Apple hardware is a violation of Apple's software license terms, so actually doing this might be illegal, depending on where you live and what the courts decide. And it is likely that Apple will update Mac OS X in some way to make it incompatible with an EFI-X-equipped PC. Nevertheless, it is a very interesting piece of news and an even more interesting product for PC hackers to play with. (And there's absolutely nothing illegal about using EFI-X to run Linux or the 64-bit version of Windows Vista - both of which have EFI support.)