I don't kow about most people but I get very pissed off whenever I SMS or email someone for a game (e.g. badminton squash etc.) and he/she replies by telling me that he/she "ALREADY HAVE A GAME WITH WHO AND WHO" and therefore can't play.
As a matter of etiquette I feel that if you can't make it for a game when asked to, just say that you "CAN'T"... its as simple as that. Or just a "NO" will do. I really am not interested with who or where you are playing with!