Wesner Moise clues us in on a new trend for MS file formats. The new Office 12 file formats are based on XML Schema and use ZIP for packaging. The trend here is that not only will Office and Metro use it, but other apps will as well. They have defined a standard packaging format based on ZIP with XML elements that specify relationships between the ZIP parts.
You should use it for your next file format! In fact a good open source project would be to create a reader library. Each program can then wrap that low-level library with a app specialized one.
More on the format here.