I have a 2005 KHS Flite, which I am fairly sure is a Reynolds 520 frame, that I absolutely love and have morphed through many variations (the original brakeless fixie, then both breaks free-wheel, then upright gentleman's bike that can still whip, and so on). Someone tried to steel it or just decided to fuck with it, and bent the downtube/put a dent in the downtube. It's not major enough to keep me from riding it - at least I don't think it should, I could be wrong. I was wondering if there is anyway to work out the dent. Maybe from the inside out somehow? It's thumb print size into the metal, with little to no warping around the indentation.
Since you didn't put a picture, I'm going to guess it is not significantly dented enough to worry about it (in which case, I'd just leave it alone since anything else will be uglier), but you should take it to a bike shop to get a professional opinion (so everything here is obviously all at you're own risk).
For seat tubes and head tubes, you can try from the inside to press the dent out but I don't see you you could do it on the downtube.
Frame blocks (made by companies like Bicycle Research, though you could make your own if you have access to a machine shop quite easily) are one solution. They're essentially 2 machined pieces of metal which you clamp over the tube and rotate the frame to try to push the tube back into shape. An example is here. Note that this will strip the paint off that area as well.
Bondo is another solution.
The extreme solution is to replace the tube (or part of it).