There is nothing for subtitles that integrates into iDVD or iMovie.
Jubler can be used to create the subtitles. You could then use a tool like ffmpegX to convert the movie to a DVD and include the subtitles (as a separate "turn on/turn off" function like a commercial DVD). Alternatively, ffmpegX will let you embed the subtitles as you encode the movie into an AVI or some sort.
Hope this helps.