If the nick is small (*less* than, say, 1/32") you can just hone it out. But it needs to be pretty small. Otherwise you may need to take it down with a gentle rubbing of the edge on a coarse stone (I use the side of the stone so I don't put grooves in it). If the nick is larger, you might have to do some spine work to get the angles back in shape but that would be pretty radical. I've taken out small nicks pretty quickly. I stay away from larger ones unless I'm shortening the blade and carving out the chips anyway.
Here is a razor I got in a while back - I cleaned it off, smoothed the edge on a 1K stone and then honed it up. It came out pretty nice and is still in use.
Inventor of the world's first safety vibrating Kamisori with night light. Go to http://badgerandblade.com/vb/showthread.php/299465-A-milder-Kamisori: Inventor of the Weckisori - (thanks sychodelix)