I ADVISE BEING VERY CAREFUL ABOUT GETTING A CORGI INTO TERRAIN WHERE IT MIGHT SLIP BACKWARDS OR GET TANGLED IN AWKWARD FALLEN BRANCHES. I'm not sure that's what happened; it's only a guess.

Here's the trip report:
http://www.nwhikers.net/forums/viewtopic.php?t=8001192&highlight=wolffie
It was a 9-day solo backpacking trip in the Pasayten Wilderness of north Washington state -- including the northernmost leg of the Pacific Crest Trail -- cut short by 2 days.
The first 5 days were too-good-to-be-true, perfect weather, perfect scenery, easy-enough yet challenging terrain, unwordly scenery. Al climbed an 8000' peak, a 7600' peak, several smaller peaks, just like usual.
I aborted the trip on Day 6, when he lamed-up, in pain. There'd been 2-3 miles of unfriendly terrain the day before, 300 downed trees, and this might have done it. It had been a long day. He awoke not quite his usual happy self, then slowed, and finally stopped. I declared an emergency and retreated in good order to an easy trail, carrying him 17.5 miles this way. The sling is made from my plastic tagblecloth ground-sheet folded lengthwise -- much like Peruvian Indian baby-blanket -- and hooked to the pack with a small carabiner to put the weight on the pack frame. It worked quite well. This is the end of Day 7, back at the trailhead. Next time, I will have Rimadyl and Tramadol in my first aid kit.
Over a week later, he seems to be entirely OK -- the vet says no obvious CCL or back injury, as I'd feared. Our breeder had warned me to assume the worst, and keep him crated and inactive, with no opportunity at all to exacerbate the injury until we got a firm diagnosis. He'll be taking it easy for a long time.
9/23/12, he shows no ill effects at all, and can't understand why we're not playing soccer at the playground.
I am not even certain that it was his knee. I *did* observe an occasional skip/hop/limp, and I'm pretty sure it was his left rear leg, but.... back? knee? hip?
To see him apparently OK is a indescribable relief.

NOTE: I do not know what happened. My best guess is that he tried to jump or crawl over a log too high and slipped backwards, landing poorly, or perhaps just too much leaping over low downed logs. I never noticed any limping, slowness or distress on that day, and we traveled over 6 fast miles on good trail after the rough stuff.

UPDATE 12/27/12:
The outcome is neither as good as I'd hoped, nor as bad as I'd feared.
There have been 3 episodes of ephemeral but pronounced morning limping since the Labor Day backpacking trip, each after a vigorous hike or soccer game. Each time, he has "walked it out" and resumed normal gait rather quickly. He stands on his hindlegs to greet me when I come home. He chases a ball like a demon (but dogs hide pain well). Yesterday's vet visit is, "mild patellar subluxation, almost a 1 on a 0-5 scale, no onbvious CCL damage (but this might not be obvious)". He does not display persistent chronic pain, so it doesns't seem like arthritis... yet. I gotta take it easy on him. He's not gonna climb K2 or play in the World Cup. He's only 6 y.o.