Regarding the Google Summer of Code 2012 and a nice KDE application called “SubtitleComposer“, I thought about a proposal for the Kdenlive project, to implement a subtitle editor and the ability to load and export subitle files. If the core developers of Kdenlive consider to apply to GSoC2012, this idea may be a possible, supported project for the next months. Otherwise, these mockups could inspire developers already working on such functionality :)
Resize subtitle track 
Cut / split subtitle tracks into pieces 
Multi column subtitle edit 
Add subtitle track 
Popup box for editing subtitles 
Edit keyframes for every subtitle 
Add or remove specific subtitle 
Hide or show subtitle editor window 
Import different subtitle formats 
Import subtitles into your project tree 
Many options for exporting the subtitles
I agree with you that Kdenlive requires this feature. I recently completed a six minute video that was subtitled. With the current titling features, this is a lot of work. I like the ideas you have put forward here in your mock ups. I have also posted my comment of support on the Kdenlive feature request in Mantis.
http://www.kdenlive.org/mantis/view.php?id=400
Perhaps we need to get a few more people requesting support for this, so the developers allocate time and resources to it.
Would be great !
Very necessary and helpfull for me too !