I want to write a simple XULrunner application to open a Theora video and let the user to edit subtitles in different languages to that video and (yeah! you are right) save it to share to other people (maybe in a subtitle repository/page).
I think that could be a nice improvement to HTML5, have a way to embed (what a wonderful element in HTML4, isn't it?) subtitles on videos, but first of all we need subtitles to test.
Work done by others
I've seen that <video /> tag is on the near (joke!) future HTML5 specification but there is actually no sign of subtitles for that element. There is some scripts and work done by other people to let users to have video elements with subtitles and a proposal to HTML5 here (in Spanish).
Scripts and blog posts: