Artray -- you're right, it's alloy, and it is definitely corroded. I've got the telltale white powder as well. So I guess the threaded nut is seized to the bolt.
Juan -- you're right, too. Usually it's my practice to grease fasteners before assembly, but it's entirely possible I forgot or was in a hurry or something when I installed these. I can't recall any longer.
Jvan8 -- glad to hear I'm in good company, although (sadly) mine have far fewer miles than yours!
The weird thing is, the allen wrench whole in the nut isn't stripped; the entire nut/bolt assembly just spins inside the fork. I say weird, because I would assume the long threaded bolt would be part of the brake's body, and thus unable to turn, if that makes sense. Anyway, I think I'll see how it likes a drill bit. The fork is toast anyway if I can't get it out, since the brake body is now too loose in the fork to be useable but still the bolt won't disengage. But maybe with some careful attention of the hand drill I can remediate the problem and get the brake off the bike. I'll destroy the brake if I succeed, but I was planning on replacing them with something else anyway so it's no great loss. I'll report back once surgery is complete.
If you can cut the axle bolt off and get the brakes out do it ,,,you can get titanium replacement axle bolts and nuts here http://www.pro-bolt.com/
" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false; [if only i knew] also you need to check the rest of your bike for dust ,because it spreads ,cranks especially,if there is more dust your bike shop or a bike shop can clean out the inside of your frame , ,,,the reason you nut spins is because the dust is very accurate and has no effect apart from where it makes contact ,,,,artray