This release makes it possible to have multiple instances of web3 running and using multiple providers.
Therefore you need to instantiate now web3 yourself (except when you use mist later on)
var Web3 = require('web3');
// create an instance of web3 using the HTTP provider.
// NOTE in mist web3 is already available, so check first if its available before instantiating
var web3 = new Web3(new Web3.providers.HttpProvider("http://localhost:8545"));