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Here's what could happen if somebody controls 50% of the nodes-that-are-accepting-connections:
I join the network with a node behind a firewall (so I only connect out to 8 nodes). I have a 1-in-256 chance that all my connections are to the cheater's nodes.
If I am very unlucky and all of my connections ARE to cheater nodes, then they can:
+ refuse to relay my transactions to the rest of the network
+ refuse to send me new "block created" messages
+ send me only the addresses of other cheater nodes
They could also send me a bogus version of the block chain (well, bogus back to the last compiled-in checkpoint), but that would be easy to detect (number of blocks in the chain would be different from what everybody else sees).
They could NOT:
+ modify/"damage" transactions
+ hurt the vast majority of network nodes that have at least one connection to a non-cheater
I'm glossing over some details and possible attacks (e.g. if the attacker can hijack my internet connections -- maybe I'm using a wireless network at a cafe -- then I'm screwed because they can just redirect all of my connections to cheaters).
The bottom line is: make sure you have the right number of blocks (bitcoinwatch.com, for one, will tell you the block count). Don't do financial transactions on untrusted networks. And if your transactions don't go through after half an hour, restart bitcoin to connect to a new set of peers.