Oh, and on the subject of 5.1 AAC downmix, there's another handbrake thread with a patch:

http://forum.handbrake.fr/viewtopic.php?f=12&t=14084

and a blog post regarding this in ffmpeg:

http://muzso.hu/2009/02/25/downsampl...ls-with-ffmpeg

However, you've got to patch and build your own Handbrake in either case