ForkSplitterConduit.sol

contract ForkSplitterConduit {
    //Tracks whether hard fork is effective on this chain. True means the fork is passed, false it hasn't.
    bool forked = false;
    //Identifies on which network fork this contract should do transfers. True transfers only on a hard-fork network, and false on the original network.
    bool transferOnlyFork;
    //Hard-fork DAO withdrawal contract
    address constant C = 0xbf4ed7b27f1d666546e30d74d50d173d20bca754;
    
    //In Constructor you set whether you want this contract to operate on hard fork or non-hard fork network
    // Set to true for transfers to only complete on hard fork, and false for non-hard fork
    function ForkSplitterConduit (bool _transferOnlyFork) {
        transferOnlyFork = _transferOnlyFork;
    }
    
    //Needs to be called as close as possible to block 1920000, can use an alarm service to call this
    function checkFork() returns (bool) {
        if (msg.value != 0)
            throw;
            
        //Checks whether it is after the block that the hard-fork is supposed to take place, which is block 1920000
        //Gives someone ~12 days days to call this, which checks whether the balance in the hard fork withdraw contract
        //is above a certain threshhold (9m in this case) which indicates that the hard-fork is live.
        if (block.number >= 1920000 && block.number <= 2000000 && C.balance >= 9000000 ether)
            forked = true;
            
        return forked;
    }
    
    //Transfers are only possible after the hard fork is supposed to take effect.
    function transfer(address _recipient) returns (bool) {
        if (transferOnlyFork != forked && block.number < 1920000)
            throw;

        if(!_recipient.send(msg.value))
            throw;
            
        return true;
    }
    
    function() {
        throw;
    }
}

Archived article here: https://web.archive.org/web/20190501000000*/https://medium.com/@timonrapp/how-to-deal-with-the-ethereum-replay-attack-3fd44074a6d8