If you have a moment, take some time to read through Chris Brumme’s entry about Application Compatibility. I’ve been staying away from directing people to information pertaining to PDC, Longhorn and Whidbey, however, this one is a must read and effects everyone equally. There is also some historical information discussing some examples of ‘breaking’ changes between 1.0 and 1.1. Even more on how Microsoft is working to use automated testing procedures to catch breaking changes. It’s a long read, but I guarantee you won’t be dissatisfied.