Is Visio DataDiagramML also part of the Open XML and does Visio 2007 support DataDiagramML as the default file format?
question 1: No, Visio is not part of Open XML. It might become part of it later in Office Open XML 2.0 or something.
Question 2: I have not a clue. Try and check the options dialog. Word / Excel allow you to set the default save format, perhaps Visio as well. Word uses a special Save panel for this.
Hope it helps, Wouter