WWJBD is an acronym for What Would Jack Bauer Do? Jack Bauer is Kiefer Sutherland's character from the television program "24." He is a Grade A Terrorism Fighting Machine, and will stop at nothing to defend the United States of America.
What, Exactly, Would Jack Bauer Do?
Presumably, Jack Bauer would do something violent, or perhaps shout a little.
WWJBD is properly used after someone has asked for advice, or when the poster feels that an action is necessary in a given scenario. Usage of this acronym indicates that the poster feels that drastic measures are needed.