I do know what a 51% attack is. Even then, it would be limited to whomever had recently sold the attackers something of value. A 51% attacker cannot steal coins that he has never legitimately possessed.
A 51% attacker can do almost anything except steal coins that have not moved recently.
Consider, for example, a 51% attacker who follows this model:
1) He buys a large number of Bitcoins.
2) He starts building his own block chain with the first block containing a transaction moving all his Bitcoins to another of his own accounts and no other blocks containing any transactions.
3) He starts depositing his Bitcoins at online wallets and exchanges. He also withdraws them, depositing them in other services, and then withdrawing them from those. He gradually begins to turn his Bitcoins into Litecoins, Liberty Reserve, and the like, using as many Bitcoin transactions from as many services as possible.
4) Many Bitcoin wallets and services begin to hold large amounts of Bitcoins that trace back to his Bitcoins. Other people who withdraw from online wallets and exchanges get Bitcoins the attacker deposited.
5) The attacker now releases his longer chain with no transactions but the one that invalidates all of his deposits to the public. Now, a significant fraction of Bitcoin withdrawals and exchanges find themselves undone. Whoever held anything traceable to his original Bitcoins, now widely distributed through the Bitcoin ecosystem, finds their balances drop.