In a recent thread another guy fixed his numb toe issue by adjusting his cleats to allow his shoe to move more outwards. His theory was he naturally pedals that way and when his shoe could not move any further outwards his feet inside the shoe would move causing his big toe to slam against the inside of the shoe.

The most common cause is restrictive blood flow to the toe due to pressure being applied to the nerve feeding the toe. Like others have suggested moving your cleat backwards should fix this.