There is an app in App World that let's you turn subtitles on and off (after you have added the subtitle file to your PB as well, of course), but it is a paid app and I haven't purchased it so I can't tell you how well that works. http://appworld.blackberry.com/webst...44932/?lang=en
Personally, I do what Psych1 does; convert my files and get the subtitles hardcoded in there. I use this handy little converter on my pc, called Freemake Video Converter. And there's a lot of people here on the board who use Handbrake for their video converting.