Re: Iron Man 3 Discussion/Photos/ETC.
I doubt it's done in post. A good rule of thumb would be to ask if you can do something practically cheaper than you can with CGI in post. It'd be a lot easier and cheaper to simply put something under his shirt that glows than it would be to every single frame of film he's on and add in a glowing reactor that looks to be under his chest.
Just because it's futuristic doesn't mean it's practical.