Looks like it's just a frontend
Which would be smart, actually. As people keep bringing up, you can simulate most of what Project does with spreadsheets and email. It makes sense that they would create a file format that is essentially a config file for how to import data from various Office apps, and then how to use other Office components to display it.
===
Microsoft offers them the one thing most business people will pay any price for - the ability to say "we had no choice - everyone's doing it that way." -- [link|http://z.iwethey.org/forums/render/content/show?contentid=38978|Andrew Grygus]