Couple of updates on this:
I was asked yesterday what happens if you write a doc in Word 12 using Calibri, save it as 97-2003 format and send it to a 2003 user. Will they see the document in the right font? Just tested this and it must imbed the font in the document because it turns up in calibri in Word 2003. If you close the document in Word 2003 and open a new one though, calibri is not available so it was not installed.
Also in beta1 anyway, if you start a new document in Word 12, the default font is still Times New Roman. Calibri and Camria are pulled out at the top of the font list though. My understanding is that this will change for the released version though. If I find out more I will post it.