Ok, I believe I resolved issues with the payment details on the Livetest. I'm struggling with Drupal's Forms API in a contest over who gets to validate what data when, and it wants to validate required fields first. That's unfortunate, because it will cause the payment pane not to expand when both the payment pane and a required field from somewhere else on the form fail validation. It's cool, though, because at least we're preserving the data... just a little bit annoying. And of course, this can be avoided by a site not using collapsible fields if they really don't want the hassle for their customers.

So, test it out and I'll be committing this for beta 5.