re: Predict the 12/31/2014 bitcoin price (plus 2013 year in review)

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But still, how does someone possessing more than 50% of the "Mining" power affect all of the bitcoins that have already been mined?

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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. This has the potential to double-spend transactions that previously had already been seen in the block chain.
-Prevent some or all transactions from gaining any confirmations
-Prevent some or all other miners from mining any valid blocks

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

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