Hello,
today I played little bit with my XBMC Media center on XBOX (also available for Mac OS, Windows and Linux) and I found nice OpenSubtitles plugin.
How TO install it:
1. download, extract and copy XBMC Zone Installer to Plugins/Programs
2. Go to Programs/Program plugins and run XBMC Zone Installer
3. Navigate to Plugins/Video and select OpenSubtitles (plugin) and confirm install
4. Go to Video/Video plugins and select OpenSubtitles ("right" click for options, you can select default languages and so on), navigate to videos and download subtitles
Thanks to Dan Dar3 for making this plugin, he also post some nice screenshots.