Although few people would use the default HTML rendered by Rails it would be a nice touch if it was compliant with W3C recommendations . In fact, most of it already is compliant. It does however miss a proper doctype and have some other minor errors.
If the rendered HTML was according to a W3C recommendation (e.g. Xhtml 1.0) my guess is that it could be of marketing value and contribute to a more widespread adoption of Rails.
Oh, and this should probably be implemented before the DesignedScaffoldingWish to make sure everything is playing nicely together.
I strongly second (and third!, &4th) this wish.
One of the main reasons (other than the pleasure of learning) that I undertook to write my own Wiki was that none of the Wikis I looked at could generate XHTML 1.0 Strict, and this was important to me.