Replay-safe refunds for everyone.

refund.sol

contract Owned {
  address owner;

  modifier onlyOwner {
    if (msg.sender != owner)
        throw;
    _
  }

  function Owned() {
    owner = msg.sender;
  }
}

contract SplitProof {
  bool notMyChain;
  
  function SplitProof(uint blocknumber, bytes32 blockhash) {
    if (blockhash == bytes32(0)) {
      throw;
    }
    if (blocknumber == 0) {
      throw;
    }
    if (blocknumber <= block.number - 256) {
      throw;
    }
    notMyChain = (block.blockhash(blocknumber) != blockhash);
  }
}

contract DaoInterface {
  function transferFrom(address _from, address _to, uint256 _amount) returns (bool success);
  public uint totalSupply;
  function balanceOf(address _owner) constant returns (uint256 balance);
}

contract Refund is SplitProof, Owned {
  uint public totalSupply;
  bool locked;
  DaoInterface theDao = DaoInterface(...);
  
  function Refund(uint blocknumber, bytes32 blockhash) SplitProof(blocknumber, blockhash) {
    totalSupply = 0;
    locked = false;
  }
  
  function lockSupply() onlyOwner {
    totalSupply = this.balance;
    locked = true;
  }
  
  function () {
    if (notMyChain || locked) {
      throw;
    }
    return true;
  }
  
  function refund() {
    if (!locked || notMyChain) {
      throw;
    }
    
    var balance = theDao.balanceOf(msg.sender);
    var scaledBalance = balance * totalSupply / theDao.totalSupply();
    
    if (balance == 0 || 
      !theDao.transferFrom(msg.sender, address(this), balance) ||
      !msg.sender.send(scaledBalance)) {
      throw;
    }
}