I have seen this question being asked in news group often. It's something very trivial to do in BizTalk map using the value mapping functoid, which returns the value of the second parameter if the value of the first parameter is true. Hope this picture will explain how to do it.
Note: make sure the logical existence functoid is first parameter in Value Mapping functoid. Sometime based on how you dragged the labels, the order might be wrong.
Nandri
Saravana