I watched the live feed of the event, and while the confusion may be understandable (given that live blogs may have missed this nuance), App Store apps won’t be single-tasking nor necessarily full-screen. In the demo at the event, for example, they installed Pages (from iWork) via the App Store.

Where the full-screen bits come in is that the green “maximize” button will run apps full-screen under Lion. App might need to be updated to support this (?), but that wasn’t clear. Either way, the full-screen-ness is a feature separate from the OS X App Store.