I don't have an answer just would like to add my own question. Since I also can not find a way to quit an app (one of the games included in the Win 8 Dev Preview full install with tools for example) if I go back to the Metro interface using the Windows key, does the app automatically close?
Actually this is not the answer, but if you want to see what apps are running on the background, just move the cursor to the left side till it touches the edge. You'll then see the popup program's thumbnails and may click it.
You cannot close a Metro application. You just press the start button from on-screen keyboard or hit alt-tab or do the same previous poster said. It's working as designed. it will be in suspend mode and if it's not used for a while it will release the Memory or if computer requires additional memory it will release the Memory. It's like a Android or W7P etc. So far I have noticed it works great very effective Memory management.