We do have a historical precedent of how the community tends to respond:  in block 74638, or alternatively on August 15, 2010, someone exploited a loophole in the then-current client to give themselves 184 billion bitcoins.  An update to the Bitcoin software was immediately created that deleted that block and corrected the blockchain while including all other legitimate transactions (the cryptographic security of bitcoin makes it easy to keep valid transactions in these scenarios), and that new version of software was released to the public.  The majority of the nodes switched immediately for obvious reasons, and the fixed blockchain overtook the compromised one at block 74691 (very quickly).  This is also an excellent demonstration of why simply trusting the most computationally intensive blockchain is a safe move--once a clear majority of computing power has switched that blockchain will always win out to become the most secure record.  And even in the midst of this commotion an attacker with more computational power than the whole network couldn't have spent other people's coins or altered their transactions.  It also demonstrates how Bitcoin's nature allows anyone connected to the network to hold it accountable.  Go jgarzik!