The current Pro Tools and Logic versions can do up to 7.1 or 7.2, I believe. But these will just give you the final product of individual tracks for each channel. (Ex. a 5.1 project will give you 6 individual files, when it's all bounced.)
A DVD-Video creation program would then have to take those tracks and encode them to a 5.1 ac3 stream for the disc. So if you are working on audio for a 5.1 project, you can do it all in Logic or Pro Tools, hearing it in that program only in 5.1, then bounce the individual channels to give to the Final Cut editor.
Dolby has recently announced software (OS X only) that lets you decode an ac3 stream. So you can open up the 5.1 streams from a DVD and get the individual channels. I saw them demo this at the GDC in San Fran in March. I can't remember the release date or any other info. But it is probably pricey.
BT's Binary Universe is a 24bit/5.1 dvd. It's pretty cool!
And I think windows media (I know, bleh!) has a 5.1 lossy codec.
Ok enough randomisms from me for the moment...