In the context of discussing IE7 and Window Vista, the Microsoft RSS team declares:
"We will only support feeds that are well-formed XML."
"I think it is a good move and saves a lot of development headaches. Feed publishers should periodically check their feeds using tools such as Feed Validator to discover any potential problems and evaluate some of problems revealed by warning messages. Errors in your feed might result in your message not being delivered to your readers."
"This is the right thing to do, and I'm glad you're doing it - thanks."
"Gutsy, and welcome, move."