In the context of discussing IE7 and Window Vista, the Microsoft RSS team declares:

"We will only support feeds that are well-formed XML."

Technorati's Nial Kennedy likes the move:

"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."

Nick Bradbury (of FeedDemon and NewsGator) gives a thumbs up too, who writes in the post's comments:

"This is the right thing to do, and I'm glad you're doing it - thanks."

Soren sees irony however:

"Funnily enough, their own Atom feed does not validate, according to feedvalidator.org (the RSS feed does)."

Sam Ruby:

"Gutsy, and welcome, move."