Subtitle editor for OSX?
Never tried this, but is is cross-platform: http://www.aegisub.org/

After i read your comments - IMHO you will need:


Make .srt subtitles in any tools what you can manage to work (timing)
Optionally repair .srt files in any texteditor - skip this if not needed
Test the .srt subtitles with VLC or Quicktime/Perian
If everything is OK - embed subtitles into video with http://code.google.com/p/subler/