Originally posted by AttilaTheHorn
The real reason for not announcing check is that it's against the rules, because you are not allowed to say anything to your opponent. The only time you can speak is to offer a draw or resign. Otherwise, you must not say anything at all or act in a way that might disturb or annoy your opponent. All this is covered in the rules of chess.
I've never played chess under strict FIDE rules, so I'm not familiar with how FIDE arbiters interpret the "announcing check" issue. Maybe Attila and Fabian have a better handle on FIDE rules than myself. However, I will say that I checked the FIDE rules on the FIDE web site, and I couldn't find any rule that specifically addresses the issue of announcing check.
Here in the USA, the USCF rules do address the issue.
USCF Rule 12F: "Calling Check Not Mandatory. - Announcing check is not required, and is rare in high-level tournaments. It is the responsibility of the opponent to notice the check, and a player who does not may suffer serious consequences. A player may announce check."
I'm told that in USCF tournaments with young scholastic players, it's fairly common for them to announce check. However, in most adult USCF tournaments, the players will get irritated if their opponent announces check, because they are good enough to know that check has been given. So in these adult tournaments, check is almost never announced.