Interestingly Windows 8 Consumer Preview includes an app called "Windows Reader" that can read PDF files. Unfortunately it seems to be a metro-only app, so if you want to read your PDFs without surrendering the entire screen to them, you'll still need third party software.
http://www.winsupersite.com/article/windows8/windows-8-feature-focus-windows-reader-142710on a related note, the old win-D (show desktop) shortcut now serves another purpose - it will take you back to the desktop no matter where you are in the Metro interface.
