I've had a few questions from folks asking how people will be able to access and program against these files. As I've said (and I'm sure you're all aware by now), in it's simplest form it's just XML and ZIP. Most of the questions have been specifically around what OM support will be provided for opening the ZIP files. If you read the developer whitepaper, you'll see that there is actually a convention we use for storing the different XML parts within the ZIP and relating them to each other. It's a pretty simple concept, and there will be plenty of documentation around it.
The conventions used for storing parts, creating relationships, and addressing other parts via relationships is actually part of a shared convention that we use in Office, and it also used by the folks that created the new Metro format (I think it's called the open XML Paper Format). If you look at part 1 of the latest Metro spec, you can see how the relationships are created, how parts are addressed, etc. This leads me into how you can program against these files. Since it's just ZIP and XML, you can of course read the specs and navigate the files yourselves. Another option is to install WinFx (currently available in it's beta form), which will allow you to use System.IO.Packaging in the WinFX SDK to open and manipulate the format. I'll be showing a bit of this at my TechEd session this week (Tuesday @3:15).