You only get a yellow if its a political (not just about politics) statement i think, if i remember it right Mad Mario didn't get one for his "why always me" on his undershirt, apologises if i wrong like.
Nah, in theory you always get one.
Ahhh i guess its a down to the ref thing, i remember Billy Sharp not getting one (rightly so imo) for his "for you son" undershirt thing.
Referee's are clearly erring on the side of caution when the message is particularly sensitive. Could you image the outrage if Billy or Cahill had been booked? The referee would have been be hung, drawn and quartered by the majority of the nation. Of course, the players really shouldn't be putting the referees in such a position in the first place...