Many identical transactions with different transaction IDs are being relayed on the network from an unknown source. As a result, a certain number of services like Bitcoin exchanges and wallets have been found to handle these transactions improperly and are experiencing temporary issues with their internal accounting systems. Transaction malleability is a known issue and many services are mostly unaffected.
What is being done
Bitcoin developers are collaborating with known affected exchanges to fix their internal systems and help resuming withdrawals as soon as possible. Some pool operators have been reported to work on blocking duplicate transactions to help mitigate the problem. Emergency fixes to the Bitcoin protocol are not required and future improvements are already being discussed.
What you should do
No action is required to protect your bitcoins. Always remember to wait for one confirmation or more before spending your bitcoins. You should monitor your own exchange's website to be informed about their service status. Temporary confusion, delays and price volatility can be expected.