Payability

Starting from Solidity 0.4.0, every function that is receiving ether must use payable modifier, otherwise if the transaction has msg.value > 0 will revert (except when forced).

Note

Something that might not be obvious: The payable modifier only applies to calls from external contracts. If I call a non-payable function in the payable function in the same contract, the non-payable function won't fail, though msg.value is still set.