Metro is irrefutably beautiful, but the imminence of Microsoft's bold new UI in Windows 8 has a lot of power users worried about just how they're going to be able to function with the new system. Judging by how multi-display systems seem to be working, the answer seems to be "more or less the same."
You'll be able to use Metro on one monitor and the classic desktop view on another. Or Metro on two, classic on one; or Metro on one, classic on three; or any other permutation you can dream up. Desktops can now be customised more fully — like having more than one image displayed, smart-selection for portrait-style monitors, and spanning a single image across multiple displays. The taskbar also functions more smartly. You can decide to have your icons always open on your main screen, or have them on the bar where the app is open.
Overall, it looks like you'll be able to manipulate your experience into just about anything you want. Which should go some way to relaxing the fears that users who want no part of Metro will be crammed into Microsoft's brave new world. For those backward-looking UI Philistines, Metro can be subverted to either nothing at all, or a glorified start menu, sequestered away on a monitor in the corner. [Windows 8 Blog]