|Photo from Flickr user Kaptain Kobold. Daleks are the best.|
Image of Doctor Who figurines being menaced by a toy Dalek.
I only found a couple mentions of this on Google but I think it's really interesting. I guess it all comes down to the personal style of the person or company handling the captions/subtitles. Remember that story last year about the company that wanted to create a sarcasm mark, the SarcMark, which would cost money to use? It reminded me a bit of that.
Are there any other tone indicators used in subtitling or captioning? (!) is the only one I can think of.