RSS Validator.
I wish I could say that my RSS feed validates, but alas, it does not. I guess it doesn't like HTML in a site's description. I don't know where I'm getting the language error from, Dave doesn't appear to get it.
Update: I updated my website description in Roller to be text with no markup and hard-coded the HTML-rich version in my page template. Now I'm complying with yet another web-standard, yee haw!