... as will "If it was your business, you'd know the answer already." (Is that last one e-hell approved in appropriate situations?)
I have used this one, but only as part 3 of a 3-step process:
1) Oh, why would you want/need to know that?
2) I'd really rather not discuss this. *change subject*
3) Really, if that were any of your business, you wouldn't have to ask.
I'd love it if someone told me if that works within etiquette guidelines or not. If not, I promise to be suitably embarrassed at my behaviour, and try to mend my ways.