Update from 2010-10-20:
This project will be closed (or expanded) as there is another more advanced project Periscope that will be ported (I hope):
http://code.google.com/p/periscope/issues/detail?id=47

This is a port of nautilus script for easy subtitles download.
To get subtitles you just need to select a file [and language].
Also usable in CLI - exec it with filename as the first argument.

Test it