Work perfectly on testnet.

I'm using the stripCrLf script at How to load Solidity source file into geth.

Here's your code, slightly tidied, saved into MyEscrow.sol :

pragma solidity 0.4.2;

contract Mortal {
    address owner;

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

    function kill() { if (msg.sender == owner) suicide(owner); }
}

contract MyEscrow is Mortal {
    uint numEscrows;

    function queryNumEscrows() constant returns (uint) {
        return numEscrows;
    }

    function start() returns (uint) {
        numEscrows++;
        return numEscrows;
    }
}

I ran the following command to flatten your code:

user@Kumquat:~/ESE/Escrow$ echo "var myEscrowSource='`stripCrLf MyEscrow.sol`'"
var myEscrowSource='pragma solidity 0.4.2;contract Mortal { address owner; function Mortal() { owner = msg.sender; } function kill() { if (msg.sender == owner) suicide(owner); }}contract MyEscrow is Mortal { uint numEscrows; function queryNumEscrows() constant returns (uint) { return numEscrows; } function start() returns (uint) { numEscrows++; return numEscrows; }}'

I executed the following code in my geth --cache=1024 --testnet console command line:

> var myEscrowSource='pragma solidity 0.4.2;contract Mortal { address owner; function Mortal() { owner = msg.sender; } function kill() { if (msg.sender == owner) suicide(owner); }}contract MyEscrow is Mortal { uint numEscrows; function queryNumEscrows() constant returns (uint) { return numEscrows; } function start() returns (uint) { numEscrows++; return numEscrows; }}'
undefined
> var myEscrowCompiled = web3.eth.compile.solidity(myEscrowSource);
undefined
> personal.unlockAccount(eth.accounts[0], "MyVerySecretPasswordNumber1");
true
> var myEscrowContract = web3.eth.contract(myEscrowCompiled.MyEscrow.info.abiDefinition);
undefined
> var myEscrow = myEscrowContract.new({from:web3.eth.accounts[0], data: myEscrowCompiled.MyEscrow.code, gas: 1000000}, 
  function(e, contract) {
    if (!e) {
      if (!contract.address) {
        console.log("Contract transaction send: TransactionHash: " + 
          contract.transactionHash + " waiting to be mined...");
      } else {
        console.log("Contract mined! Address: " + contract.address);
        console.log(contract);
      }
    }
  }
)
Contract mined! Address: 0x3fd50995536919085c28fdf9e07a03bad955cc33

myEscrow.start(eth.accounts[0], {
  from:web3.eth.accounts[0], 
  data: myEscrowCompiled.MyEscrow.code,
  gas: 1000000
});
"0x0b2ec2ee98b21214cca5be9294f3c019239d3d90f6e23c03a3eb7d6fcc8c1c29"
> myEscrow.queryNumEscrows()
1

Your code is working perfectly on testnet. Check your deployment process.