I'm not using it right now on an MBA because I'm back on a MBP, but I did try it out for a few days on the MBA using Fusion and XP. Performance was very good, snappy and as reliable as it is on any PC. I've been using DNS for a few years first with bootcamp and then Parallels/Fusion, but I have to say what I've longed for is something equivalent on the mac side since I like working in OS X. I think the Macspeech people have finally introduced a product that eventually will be as good as DNS - the old macspeech stuff was appallingly bad in comparison. As I understand it, you can't edit in the new version using speech, but I presume they'll add that eventually and I'd wait until they do. It's a kind of strange omission in my view, but I guess they were eager to get it out the door.
So: yes, DNS works fine on a MBA with a USB microphone. My advice is to spend the necessary money on a high quality mic or you'll be frustrated. With DNS, I've used theBoom and plantronics wireless Cs50-USB , both of which are excellent in terms of accuracy and noise cancellation. The quality of the headset is extremely important!