NFPA 13 would be the reference needed for your question. Since you state its a concealed light fixture, I'm going to say there in no requirement. Location requirements come into play when a light fixture or other object can obstruct the spray pattern of the sprinkler. As long as the light fixture does not obstruct the sprinkler spray pattern, your fine. I would try to keep it a couple inches away for aesthetics.
if you have a light fixture that protrudes below the ceiling, it's a different ball game.
I concur with moopey. While working as a commercial electrician, this type of question is often discussed. As moopey stated, since there is no concern with blocking the sprinkler, it should be a non-issue.