Huge improvements in multi-monitor support for Windows 8 Release Preview
It looks like there's some huge improvements in multi-monitor support. It's covered in Neowin http://www.neowin.net/news/huge-multi-monitor-enhancements-coming-in-windows-8
The biggest improvement of course is hard corners EVEN with multiple monitors. Now, your mouse cursor won't slip to the next screen when you hit any of the four corners. This is one big issue that I have with the CP version.