As an insider. It is true Microsoft will offer Windows 8 as 64bit NO 32bit option. As many people have pointed out 32bit has been around forever and it is time it goes the way of 16bit.
This is a joint venture with Intel. Intel is stuck at about 4GHz for processors because they are not able to make the high powered laser that makes the chips make them any smaller (I know the guy that loads the silicon plates into the laser). Intel needs to go smaller to make the chip go faster ... or do they??. Moving to 128bit processors means you can stay at the same circuit size and even though you are still at 4GHz your pipe is double, this give you an amazing speed boost.
A Microsoft engineer leaked that Windows 8 will most likely be released as 128bit as well. Microsoft will not release Windows 8 as 32bit, 64bit and 128bit way too much coding and compiling to keep up with, plus all the coding issues with 32bit drivers. Not to mention updates. Microsoft is on a hard push to have all newly purchased machines be shipped as 64bit unless the user explicitly requests 32bit. Currently most if not all machines that are 5 years old or newer support 64bit. My 5 year old Dell runs Windows 7 64bit perfectly. Windows 8 128bit, will go along with the Server Line (Windows Server 2012 being 64bit and 128bit). Microsoft did a great job of redesigning the way drivers work so that 64bit or 128bit are extremely transparent and easy to code. Unlike all the restrictions in the 32bit model.
Edit: Forgot 32bit and its ~ 3.5 Gig of RAM issues. Now-a-days everyone wants 4,5,6 or 8Gigs of RAM so 64bit is the way to go.