I really appreciate both of your replies. Thanks very much.
PDavis responded by email; his solution was to put the cassette back on after removing the axle cap, which gave him enough grip to pull off the freehub. This will be the first thing I try.
Andrew, your suggested solution implies that normally, the axle is free to slide out from the non-drive side, and the only thing holding it in it the drive-side axle cap. That seems unlikely, but if the cassette trick doesn't work, I can certainly tap it such that I won't break anything, but will learn if it will move.
Again, thanks to both of you for your suggestions, I really appreciate it.
I have a brand new Aeffect rear wheel, and I am having the same problem. As described above, I pulled off the non-drive-side axle cap (which slides on and is held by an O-ring), used a 12 mm Allen to hold the axle and removed the drive-side, reverse-threaded axle cap. (Andrew, you have it flipped in your Oct. 27 reply.)
I think the freehub should slide off, but it is solidly stuck on the axle. I've even tried putting the hub body in a vice (with plastic jaws) and pulling up on the wheel, but I can't get it off. If you have a solution, I'd love to hear it.