What can an attacker with 51% of hash power do? Suppose organization X has 51% of the hash power for a period of 1 week. In this week, what exactly can and can't X do?
Actually, it's very easy to do damage to the network once you have 51%; just build your own chain faster than the network, and broadcast it whenever you like. If you send some of your coins to a new address in your own chain, all the transactions issued in the live network by spending those same coins will be reversed at the moment the longer chain is broadcast.

Right from the bitcoin wiki (probably proof-read by many pairs of eyes) :


  An attacker that controls more than 50% of the network's computing
  power can, for the time that he is in control, exclude and modify the
  ordering of transactions. This allows him to:
  
  
  Reverse transactions that he sends while he's in control
  Prevent some or all transactions from gaining any confirmations
  Prevent some or all other generators from getting any generations 
  
  
  The attacker can't:
  
  
  Reverse other people's transactions
  Prevent transactions from being sent at all (they'll show as 0/unconfirmed)
  Change the number of coins generated per block
  Create coins out of thin air
  Send coins that never belonged to him 
  
  
  It's much more difficult to change historical blocks, and it becomes
  exponentially more difficult the further back you go. As above,
  changing historical blocks only allows you to exclude and change the
  ordering of transactions. It's impossible to change blocks created
  before the last checkpoint.
  
  Since this attack doesn't permit all that much power over the network,
  it is expected that no one will attempt it. A profit-seeking person
  will always gain more by just following the rules, and even someone
  trying to destroy the system will probably find other attacks more
  attractive. However, if this attack is successfully executed, it will
  be difficult or impossible to "untangle" the mess created — any
  changes the attacker makes might become permanent.