Again, use common sense when making judgement calls -- the moderators will do the same. Sarcasm is definitely allowed, as long as the things you say aren't insulting nor rude.

Though, don't forget this is a written message board; sarcasm is very difficult to convey through such a "basic" medium. I for one should know, considering that more than one of my posts have been reported to moderators for being offensive, when really it was sarcastic (and I was even using the ":P" smiley, which indicate the tongue-in-cheekness of the sentence at hand).

Also, it is very difficult to imagine the shoes in which another user is standing. Sometimes they are really fed up with things, and misunderstood sarcasm may ignite a whole series of things no-one wanted and no-one even imagined. Be careful, and assume good faith.