Windows 8 is not only including a tile-based shell for tablets called "Immersive", but there will also include "AppX": a new type of packaged application model in Windows 8 that resembles Windows Phone 7 application packages.
This would then mean the AppX application type could be common between Windows 8 and Windows Phone 8, this provides developers with a way to write applications for both platforms. This would also allow Microsoft an avenue to have apps working on two Windows operating systems on a variety of different devices like desktops, laptops, tablets and phones. This is pretty exciting stuff!
Attributes of AppX include (but are not limited to):
Application identity - name, publisher, version
Application architecture - processor architecture, type of application, framework required, operating system version
Dependencies - name, publisher and minimum version of other required applications
Capabilities - networking, file system and profile capabilities requested by the application
OS extensions - associated filetypes and protocols, AutoPlay, "Charms," notifications, splash screen
Tile customization - logo, name, description and colors for the tile-based user interface