My advice? Wait for the vet (hope it's a lameness vet who specializes in lameness diagnosis and treatment, because not all vets are good with lameness). If you ask on the internet you'll get a different answer from every single poster, none of them are likely to be right, because they can't see the horse or put their hands on him...and most of them aren't vets, either. They'll just think it's something they've run into before.
There is absolutely no way anyone can tell you what it is from your description - even if you supplied a video. Lameness is just not something anyone can diagnose over the internet. 'It's the foot', 'it's the shoulder'. There is no point. The vet needs to get his hands on it and do a good workup/diagnosis.
Lameness is a change to the stride due to pain. The pain can be just about anywhere. About 75% of lameness in the front legs is caused by a problem below the fetlock. About 75% of lameness in a hind leg is in the hock. Your horse could be in that 75%. Or not. The vet will sort it out.
If the horse is looking ok, then every few strides takes a bad step, it is in pain. It needs to not be worked and not turned out unless it is a very small area where it will not run, turn, twist, or get chased by other horses.
If a horse is still lame like that after a course of bute, I honestly would be very concerned; I'd be taking the horse into the clinic for a complete lameness evaluation. If the guy you have coming out Wednesday is a good lameness vet, that's good.
Flexion tests and xrays are generally used to diagnose lameness. I'm not a fan of nerve blocks. That's a lot of pain and swelling (and guessing) to put the horse through because the vet doesn't have a portable xray machine that would sort it out a lot more accurately.
There are a great many causes of lameness that involve no swelling, no heat, and no obvious signs other than the lameness.
Arthritis, ligament injuries, an abcess that just doesn't create any heat you can feel on the surface of the foot, etc. Lots of different possibilities.
Last edited by slc; 02-15-2011 at 08:45 AM.