When people can't hear the tone you are actually using to convey a message, it becomes difficult to spot sarcasm.
Essentially, all a recipient has to base the context of a message (in text) on is what words are used in what sequence.
I feel you though, I get annoyed when I am trying to convey a blatantly sarcastic message and find myself in the
midst of a stuff-storm of baww'ing.