The Doom9 thread is in dire need of an update.
Womble Video Wizard DVD does it automatically as well... your mileage may vary.
Stereo clips are the easiest to work with since there's usually some spatialization already present.
When I did L.A. Story all of the deleted footage was mono.
I used the graphedit method and created custom matrices (in ffdshow) and switched around between the different configurations of DPLII and DTS Neo:6 upmixers.
It gave me flexibility when I wanted the sound in the center channel, the front three or to surround the soundfield completely (sometimes all 3 in the same scene). (I also dropped in some surround audio effects I found in other parts of the movie and online).
Anyway... it all depends on what you start with and where you want to go.
The Doom9 guide mentions that ffdshow doesn't have the ability to save directly to dd5.1. This isn't true, that feature was added a couple years ago.
Using GraphEdit you can connect your file to a multichannel decoder (I used PowerDVD's not Intervideo's) and then go right to ffdshow which has an option to output ac3 to file.
Really there's a pressing need for something automated and tweakable that does this since it's complicated as all heck.
Wasn't there another similar thread here before?