"I played around with upmixing a surround 2.0 track into 5.1 but had problems with the rear channels, basically it involves putting the 2.0 through WinDVD's Dolby Pro-Logic audio processing to end up with 6 mono audio tracks (except the sub track is empty, easy to create though from the 2.0 source) which you then mux into an AC3 file, the problem I had was that I couldn't get it to do Pro-Logic II where the rear surround channels are different, I was getting Pro-Logic I results where the rear channels are identical - my A/V reciever does Pro-Logic II upmixing better anyway so at that point I gave up trying to create 5.1 from 2.0. "

That's almost what I did, except that I used Graphedit to split up the 2.0 into 6 channels, precisely because it *did* do Pro-Logic II decoding. I've gotten a lot of positive feedback on it.