Well, it's a bit early so who knows what the plan is, but as a dual monitor user, it seems like that configuration is being left out of consideration for a complete Metro experience (as someone else mentioned you could have Metro on one monitor but there was no option to use it across both monitors).

At the same time it seems that in the desktop itself the taskbar has been updated to work on multiple monitors, which is where I was going with the above post - sure they fixed the taskbar, but windows 8 is all about Metro!