Update 8/20/2017:  

It's almost 5 years later and I thought I'd add an update to this post with what has been working for me and Mishka since the last time I posted.....

Someone on the Corgi-L mailing list asked for recommendations for knuckle-dragging, so I threw together a Google site with some Youtube videos showing how I trim/make and apply nail caps and knuckle spats for Mishka:

https://sites.google.com/site/knucklespatsforcorgis/

Update 9/13/2012:  

After talking to my regular vet, I took Mishka to see an orthopedist.  She checked him over very thoroughly and basically said that all his bones and joints look about as good as they can for a Corgi (she happens to have a Bassett Hound herself, so she's familiar with dwarf dogs).  Not responding to the pulling or prodding in any way that would indicate a soft tissue problem either.  So, like many Corgis who drag their nails, that's just the way he walks, and he happens to do it enough to scrape that knuckle as well as the nails.

She talked to her physical therapist to see if there were any non-surgery rehab things we could try, and the PT (as it turns out) had just had another Corgi in with the same odd dragging issue.  Said there are exercises that can help train a dog to pick their feet up a bit better and for longer walks, a custom shoe to prevent the knuckling action.

Just went to see the PT for the boot fitting - should get the boot in the next week or so.  We'll go in to pick up the boot and to go over the exercise regimen, which we'll need a longer appt for.

So, hopeful that there can be some improvement with the exercises, hopeful that a custom boot won't rub his toes bloody, and even if neither truly solves the problem, I feel a lot better having had an orthopedist and physical therapist examine Mishka and say there's nothing really out of place - and no reason not to keep doing agility (provided that I continue to be cautious about jump height, etc. the way one should with any Corgi).

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I know a lot of corgis drag their front toenails - the various vets we've seen say it seems to be common.  What I haven't heard much about is knuckle-dragging.  I don't mean knuckling over in the back feet, an early symptom of DM - Mishka only does this on his front feet.  Well, one of his front feet.

That little black spot is actually a patch of very short spiky white fur with city dust on it - the black washes off with shampoo to reveal normal skin underneath.  When I point the toes down, the patch is exactly over a knuckle.  He is too wide in the shoulders to conform for show, and he has a distinctive swagger movement, so I figure there is some correlation between the extra width and how his legs track under his body.   It is a constant spot, does not come and go, does not change, and to the best of my knowledge Mishka hasn't experienced any trauma to his front legs.

(FYI:  The oddly discolored center nails on both feet are actually nail caps - without them we can't go for walks because the nail dragging wears them down to bloody stumps.  The caps wear through in about a week so it doesn't interfere with regular weekly nail trimming - and you can see that his fur wears down on the tops of the nails from dragging.)

So....my question for you is - does anyone else have this happen with their dogs?  Is it just more dragging in the same way that his nails drag?  He doesn't seem to be in pain and he doesn't hesitate to play normally, but perhaps it does hurt and he's just used to it?

Since this spot is only showing up on one side, it worries me.  Is that leg unhappy?  Is he not picking the foot up as much as the other one?  

What kind of veterinary specialist would I go to if I wanted to get this evaluated?  Orthopedist?  Chiropractor?