Still, what you really need is the candidates expected/required compensation, not their current compensation?

Ther are jobs I'd take at half my current salary, and jobs I wouldn't take at 3 times my current salary. In an interview situation I'm happy to provide a ballpark figure as to not waste time, but I won't provide my last salary (my tax details are public!), and I won't negotiatiate salary until I have a good bargaining position.