EFI includes a compatibility mode to support operating systems that require BIOS. It's not emulated in any sort of way.
Older Macs required a firmware update for this but current ones can be wiped out of the box and have any other x86 based operating system installed on it.
The Boot Camp Assistant is just a simple live partition editor and Apple provides the drivers for Windows.
So you could just install XP or Vista straight on to an unformatted Mac? Yes or no. I'm honestly curious to know as it may affect my next purchase.