RobinK,
I agree that seeing an opponents hand gives a player an unfair advantage over other players. However, I see no way to force a player to expose his cards to the table because another player says he saw them. Barring an extreme situation where a player is obviously not protecting his hand and exposing his cards deliberately, I do not agree with showing the cards to everyone. What if the player saw the cards, but did not see them good enough to identify them? Are we going to call the floor, tell them what you saw, then ask the player if those are the cards he has? What if they turn out to be different? This situation can only be controlled by a good dealer and players that respect the integrity of the game. I've played in, and dealt in many games where players will tell another player to "watch your cards, I can see them," or a similar statement. I've also caught glimpses of cards that were not what I thought they were.
Security for poker games is up to all who play the game. The floor, dealers and players all have a responsibility to protect the integrity of the game. If players are deliberately exposing their cards, or become repeat offenders, then drastic measures should be taken Players seeing the hole cards of others is a very comon occurence that we need to guard against. I think the intent of the players is the issue.