This won't work for all video files you may find on the web but it will work for files you rip from your own DVDs
Mobiola Video Studio allows you to rip dvds into BlackBerry format and allows for the option of enabling subtitles for your videos.
If you have the.srt files for the videos you want to watch you may in fact be able to add them to the processing list and get them working in a similar fashion.
You can get Mobiola Video Studio at BBGeeks.com for $24.95 (there is a free trial)
Mobiola Video Studio: Watch your favorite Videos, DVD, YouTube, iTunes on mobile - Entertainment BlackBerry Software
Or get a demo here as well
Download Mobiola Studio (Pocket PC)
Good luck - hope that helps.