As I understand it, EFI has a compatability module that is designed for BIOS compatability, IE it can be configured so that it will boot operating systems that requre BIOS and do not natively support EFI. However, it's up to the manufacturer of the motherboard/EFI system if the compatability module is included and to what extent. That was done do ease the switch from BIOS to EFI for system makers. It is unclear, as of this time, what features the EFI system on the new Macs will use, and if it is required to include a compatability module of this kind. It is entirely possible, if such a compatability module is included, that you will be able to load windows on a new Mac. IMHO, it would be silly for Apple as a company not to include such a compatability module. They would still be selling their machines for the same price with OSX, and the user could then pay extra for a copy of windows to install if they so desired. Also, since all the hardware in the macs is now the same as that used in PCs, I highly doubt that there will be much of a problem supporting windows on Mac hardware. Remember Steve said that they wouldn't do anything to stop you from running windows (and why would they if that's what you want to do - you're stilling buying their stuff at normal price). A microsoft rep in charge of the mac devision, and for the life of me I can't remember his name, was quote as saying something to the effect of: we welcome any new comptuers that can run windows. From this I'm guessing that microsoft will indeed support the mac hardware (as if it's much different than PC hardware).
The dev kits, which IIRC currenty use BIOS, have no problems running windows. So it all hinges on the compatability module included with apple's implementation of EFI. I don't doubt that you'll see iMacs and MacBook Pros running windows hours after they are received if the compatability module exists in a form that allows BIOS OSes. This also means that all flavors of linux for x86 would also work.
Take this for what it's worth please. I'm pretty much summing up the discussion about this from the
www.macnn.com forums and expressing my own oppinions. They are only my oppinions based on the facts that I know - and obviously if your primary goal is to run windows on a new mac you should wait for some rich sob to test it first. That being said, you could actually try OSX!