Listen and watch Jensen Harris from Microsoft who is discussing the reasoning and development processes behind the new (ish) Office 2007 Interface design and how it came about
NOTE : its split into ten minute sections so you will need to get the next one from you tube

Here is the link to part 2 where it gets more interesting :

http://www.youtube.com/watch?v=IYg647Jx7Y8

very useful !!!

part 3 :
http://www.youtube.com/watch?v=7i3E-ZfMVNQ
Jamie