Can an autobody shop still repair a panel dent if it's starting to show a little bit of rust?
Yes, they will probably pull the dent, sand it down and depending on how deep the rust is they might bondo it and then paint. That's a auto body shops specialty.
Yes, they can. If the rust is bad enough then, like Jakdragon said, they might have to bondo it. I would take it in soon though before the rust gets too bad.