> Most videos start with black 

It is even worse than that, not only do most start with a fade in many more start with some kind of opening credits (a rather obnoxious waste of bandwidth to put it right at the start of a stream).  If for example you thumbnail movie trailers you tend to get a whole lot of ugly green MPAA warnings and other junk.  

On windows I use a thumbnail browser called Xnview which includeds video thumbnailing and I'm fairly sure it uses the second keyframe to avoid this kind of thing (there may be cleverer ways to automatically pick a more useful thumbnail image to represent the stream but on balance grabbing one of the first few keyframes sounds like the best to me).  

Why not take a keyframe from the ¼ of the movie length? The start/ends are often for credit rolls/warnings, and the middle is often used for commercial breaks.

Don't forget that pitivi is a movie EDITOR, not only a viewer/converter.
When you create a new project most data will be raw unedited data. Only when you make a compilation you have to import data with credits/warnings/... I think most people will edit new data, so seeing the first I-frame can be handy.

Some sources will have default in/out points. Perhaps the thumbnail should be based on one of those (probably the in-point). If the user adjusts the default in/out point for a source (which you currently can't do, but we might potentially add support for that), then the thumbnail should refresh.

Nobody is working on this, removing the "ASSIGNED" status.

commit 5490685bce980683a09d28973e768438eda0f170
Author: Alessandro Decina <alessandro.d@gmail.com>
Date:   Sun Feb 21 15:52:34 2010 +0100

    discoverer: seek at duration / 3 when generating thumbnails.

It works. The only downside is that it will crash pitivi if encountering problems such as bug #610613