Question regarding screencast "Creating a weblog in 15 minutes with Rails 2"

Probably all of you have seen the new 15-minute-screencast-wonder over

"Creating a weblog in 15 minutes with Rails 2"

It squeezes tons of know-how in 15 minutes and shows how to setup a
weblog in rails2 in a restful way.

My question on that screencast-setup:

In the screencast, the comment-form is inside a post-view. Now let's
assume the comment-model as shown on the screencast would have

How would your create-action in the comments-controller look like in
case there would be a validation error and you wanted to return that
error to the form?

This case isn't handled in the screencast and I'm really wondering how
to solve that in a restful way.