I know this question was asked ages ago, but I'm sure others are probably looking for an answer too (as I was over a year ago, when I started working with Audition 3).
In my experience, there is no way to directly export a functional 5.1 file from Audition. You can surround mix a file using the surround panner, but you can't export in the file format used by most home theater systems (.ac3 - Dolby Surround Sound). My solution was to track down a standalone encoder, although there may be pugins available to allow Audition to do this. After surround mixing, I export to 5 mono wav files and then drop them into this absolutely ancient program called Soft Encode, which creates a single, interleaved .ac3 file out of my source files. Then, I take the .ac3 file, create a blank video the same length, and use my video editor to burn them to a DVD.
From what my research turned up, the reason Audition does not have the capability to export in .ac3 is because of the expense of licensing with Dolby. I would love to see a more sophisticated surround panner in AA4, as well as the ability to natively export .ac3 files, but by the same token, I don't want to see a skyrocketing price, so I'm waiting to see what happens!