Not that true now, Virtex has been in advance on MtGox on price move for few weeks now.. I dont think MtGox will "dictate" the market price for long.. Following both, Virtex moves before MtGox for almost a month, and has always been followed by Gox..
Maybe we will soon see a BTC price expressed by an average of a number of exchange instead of MtGox price only ! I hope so !