Dave Winer recently wrote

We added a link to a page of encoding examples for descriptions, under Elements of <item>. The change is also noted on the Change Notes page.

I was one of the people that gave feedback on making this clarification to the RSS 2.0 specification and I'm glad it's made it in. Funny enough, not even a week goes before I've had a need to forward the link to an RSS feed producer explaining how to properly escape the content in <description> elements. In this case it was the Microsoft Research RSS feeds. Its pretty clear that this clarification was needed if the folks at MSR didn't get it right the first time they took a shot at it.