I was surprised to see the following dialog after an in-place upgrade of Office 2007 to Office 2010:
Admittedly there is a strong steer towards the Microsoft formats which, we are told, are “designed to support all the features of Microsoft Office”.
On the other hand, this was an in-place upgrade and default save options were already present in Office 2007. Given that most in-place upgrades preserve settings – which is part of the point of an in-place upgrade – you would expect it just to keep the old defaults.
I’m guessing therefore that this is aimed at appeasing/convincing regulators and governments that Microsoft Office plays nice with standards.
That said, there is little reason to choose the ODF format unless it is required. It will cause problems with formatting and content, and is especially risky with Excel spreadsheets.
If you want to use ODF, save money and get more complete support by using OpenOffice.
Update: Neowin has some background here.