Sorry for bringing back old topic back to life, but just in case somebody will be searching for them: http://www.iversenit.dk/dev/ffmpeg-headers/
Also, this new tutorial to using libavcodec could be handy (C+SDL): http://dranger.com/ffmpeg/ffmpeg.html . I may rewrite it to pascal, since I'm interested in ffmpeg project too (it will be like... my 4-th console player.. heh). I hope it helps a bit.