I just happened upon a new restaurant run by a Korean guy and he cooked up some very very good, authentic tasting Pad Thai. The noodles come in chicken (5,500 won) or prawn (5,000 won) varieties and the serving size is enough to fill you up.
My only gripes were that there were no peanuts or lime juice. The owner speaks good English, and he told me that peanuts are usually in there but they'd just run out that day. Also, he reckons that Koreans wouldn't eat it if he included limes (not that you can buy them here anyway).
Besides the noodles, there are a few other things on the menu: a chicken roll, some salads and a few other things I can't remember. You can also get Cass on tap for 2,500 won. And there's bubble tea, which strangely hasn't caught on in Busan yet.
The name of the place is "Just A Taste" (edited: I go the name wrong before). Its not a place that obviously looks like it serves Thai, but you can easily see English menu board from the street.
To get there, you go out exit 1 at Kyungsung Station, and walk straight. Go past the police station and take the first right at the Full#25 convenience store. It will be on your right, a few shops down from the corner.
Definitely check it out if you're into Pad Thai - and while you're there try to talk the owner into investing in a bottle of lime juice to keep on the shelf.