Here's a question I got from someone wanting to store alternative formats in the files:

Want to know if alternative formats can also be stored in the same office XML zip format. E.g., Is it possible that the XML file format also stores the 2003 binary format as an alternative? Or a pdf version of it along?

This would provide the user another level of backup if some part of the XML format is corrupt.

Does the current Open XML schema allow such inclusions?

I have access to the beta 1, but I could not create such documents. Do you see it as a possibility in near long term (office 12 GM)

If yes, how would the Office UI react to opening such documents?

This is actually an issue we thought through a lot at the beginning of the project. One of the advantages of the packaging model we chose was that it's easy to add additional content to the files. This could be used in a number of different scenarios. One potential use would be to do something similar to the binder (binder was was an old feature in earlier versions of Office that let you take multiple files and bind them up into one "project file"). Another case (which is what this question was about) is the ability to take just one document, but embed multiple representations so that viewers could choose which format they best support.

For Office 2007 at least, we aren't planning to support either of these scenarios. That said, there is nothing in the format to prevent a solution provider from extending Office to output alternate representations. Office would have no problem opening the file either as long as the proper relationships were set up in the package. For example, someone could capture the save event, and in addition to saving the file, programmatically do a save to PDF and put that output into the package as well.