Why is 6 the number of confirms that is considered secure?
I believe this is addressed in page 8 of Satoshi's Bitcoin paper when showing the probability of an attacker catching up.

The wiki states: 


  "Only 6 blocks or 1 hour is enough to make reversal computationally impractical."  


The key word is "impractical".  

The important sentence in Satoshi's paper is: 


  "Assuming the honest blocks took the average expected time per block, the attacker's potential progress will be a Poisson distribution".  


So, it is not that someone couldn't create the worlds first and own a $50 million ASIC farm, so as to have the ability to overtake the blockchain from a fork six blocks prior but that doing so would be a.) a massive undertaking, b.) obvious to all and c.) terrifically unprofitable to the attacker.