pragma solidity ^0.4.11;

contract Factory {

  address[] public pizzas;

  function Factory() {
    // constructor
  }

  function cookPizza() returns (address _pizza) {
    Pizza newPizza = new Pizza();
    pizzas.push(newPizza);
    return newPizza;
  }

  function getPizzas() public constant returns (address[]) {
    return pizzas;
  }
}

contract Pizza {
  function Pizza() {
    // constructor
  }
}

truffle(development)> Factory.deployed().then(function(instance) { factory = instance})
undefined
truffle(development)> factory.cookPizza()
truffle(development)> factory.getPizzas()
[ '0xc4f8cf2d5a37e74981fffdab1b2108931822359a' ]

Running migration: 1_initial_migration.js
  Deploying Migrations...
  Migrations: 0x42e40188763019dd09a6166b76805d329bd71111
Saving successful migration to network...
Saving artifacts...
Running migration: 2_deploy_contracts.js
  Deploying Factory...
  Factory: 0xb9d60d9ddf16e3af3bce774a1e9286ecf5614166
Saving successful migration to network...
Saving artifacts...

> sender = eth.accounts[0]
> abi = "abi"
> contract = eth.contract(abi)
> factory = contract.at("0xb9d60d9ddf16e3af3bce774a1e9286ecf5614166")
> factory.cookPizza({from:sender})
"0xf0ce6f4c0a9665f593d3d837eca4977b85d011dcb3620bf3a2428d25225db967"
> factory.getPizzas()
[]