The title pretty much says it all. I have a 13" version of the newest-generation MBA. Can I get 5.1 surround sound? If so, do I have to do anything special to enable it?
Not natively, no. The headphone jack is a standard TRS with no optical component.
You can buy a USB sound "card" with 5.1 audio output such as this one:
That's just the first one I found, so a little more market research might be in order. Something similar to it should get the job done though.
You can also get 5.1 out with an HDMI cable and a suitable HDMI switch with optical or other 5.1 out. Note that running HDMI through a TV (or even possibly certain receivers) likely won't work, because of anti-piracy hardware that restricts output audio quality. There are a number of cheap switches that people have done this with. I don't want to suggest anything specific as I haven't used them myself. You can also get 5.1 audio out from an Airport Express's audio jack. Unfortunately the Airport Extreme has no audio out (at least not the model I have).