For those video editing buffs around here this is a piece of software you need to have: SlowmoVideo.
It is written by one of the Kdenlive developers but not included (yet?) in it.
Everything about it here: http://slowmovideo.granjow.net/docs.html
It can use the GPU if you have a powerful one and greatly reduce rendering time. Check those really impressive videos on the site.
It can also act as an video stabilization or morphing software (more or less) but also with impressive results, check this video:
http://www.youtube.com/watch?v=E0STeYT5x2Y
Note: there is already a slow motion plugin in Kdenlive, but it is not the same thing.