Universal File Formats
Via Brian Jones blog, I looked at the Metro spec to learn more about the Office file formats. The spec documents in significant detail the new universal XML+ZIP packaging file format including numerous new concepts such as part and relationships.
The packaging format is based on ZIP, but it goes beyond ZIP to provide additional features such as interleaved formats. These extensions don't actually extend ZIP but, instead, introduce numerous conventions in file naming and directory structure from within ZIP.
The primary benefit of using ZIP is its container feature. The fact that packages are based on ZIP doesn’t necessarily imply that they are compressed; packages can be uncompressed or they can also be compressed in a multitude of ways (maximal, normal, or fast).
It appears that the new XML + ZIP page file format is going to be used Microsoft-wide, not just for Metro and Office, but elsewhere like in Avalon document containers, for example.
Based on information from TechEd and the title of a new session in PDC, Microsoft will be encouraging ISVs to adopt the same format for their own applications. Widespread use of this format would make more tools available, and it would also make it easier for businesses to create, modify and consume non-Microsoft file formats. API support for this package format is found in the new System.IO.Packaging namespace defined in WindowsBase.dll in WinFX.