Is it allowed to prepare for a game after the round has started? At the recent US Open I started the tournament well and was paired against a GM in the roped-off area. I had worked out the pairing well in advance and was able to do some fairly heavy openings preparation before the game. The official pairings, though, went up a relatively short time before the round.

My opponent did not show up until about 20 minutes after the round had started. I heard one of the tournament directors remark to another that my opponent was probably preparing for the game. After the game (he had 350 rating points on me and ripped me to shreds even in one of the lines I had prepared) he volunteered the information that this was indeed what he had been doing.

While I do not think that this was unethical, I am left wondering: is this legal under USCF rules? Obviously it does not affect the outcome of the game at this point, several weeks after the fact. The reason that it seems like a potential gray area to me is that a player doing this is looking at a chess database after his clock has started.

I would ask about FIDE rules but it seems like the zero-tolerance forfeit rule would render it a moot discussion.
I would think not since there is no realistic way for the organizers to find out if you are preparing if you are in your hotel room for example, and even if they do you will probably not forfeit your game as long as you are not in the playing hall.

Maybe this was why the zero tolerance rule was put into effect.

In a logical sense though, whilst your opponent could hope to get an advantage out of the opening against you, you are getting a definite time advantage, so you could see it that way.