I would never work for a company that went to that level. In 10 years, I havent seen a company pull that. If you suspect that might be part of their hiring process. Just say "I need to make X" and evade all talk of current salary.
As I mentioned in previous comment - companies would do this in case they really want a candidate but feel that previous compensation is somewhat above the market. They OK to pay market rate (if you can prove that) but are weary that a candidate employs the strategy suggested by the parent comment. I this case showing proof of previous compensation usually helps with negotiations.