That will be the next thing I'll be working on after I've completed the current project I'm working on. I'm with the many using GDK that needs playback of videos. So far I've come up with a couple of ideas.
1. ffmpeg codec and the SDL library (or the allegro library)
2. smpeg library
I did use "vfw" video for windows with a little success but I only got wmv's working with it and it also uses software rendering.
The allegro library allows blitting to a windows hdc handle, not yet tested though.
The smpeg and the ffmpeg hopefully will render to an SDL layer.
I'll post something up as soon I get round to it.
Warning! May contain Nuts!