There's a buzz around the Microsoft URI Handlers. Basically, applications that rely on that Windows service can be handed data that isn't separated from code, which is rife with problems in and of itself.
First if affected Firefox, and there were some "no, you should fix it" comments thrown back and forth between Microsoft and Mozilla. Now it's affecting Microsoft applications.
The questions really are: Where should the data validation process occur? Should it happen just once by the OS's built-in URI Handlers, or should each application do its own validation?
The real answer is not that one side or the other should do it, but they should both do it. Any application relying on the Microsoft URI Handles is trusting that the data elements are free from anything malicious. Quite simply, Mozilla and even Microsoft product managers in charge of the apps are naive if they are not also performing validation. It's a matter of understanding trust and trustworthiness.
UPDATED: There is a slashdot thread on the subject and the Microsoft Security Response Center (MSRC) have posted to their blog explaining why they have done what they have done. Either way, if you are a third party application developer, you need to understand that it's always in your best interest to sanitize data yourself-- if for no other reason than the component you trust might not be 100% trustworthy.