"Years ago, floor traders matched buyers and sellers. It's that old Eddie Murphy movie 'Trading Places,' where men jockeyed for floor space and used frantic hand gestures to complete transactions. But today, computers dominate much quieter trading floors. High- frequency trading -- rapid, automated buying and selling -- accounts for 50% to 75% of all daily trading volume,...."
Any discussion that includes Trading Places has to get blogged!
(seriously. It is in my contract ;) )