We will soon be updating the API login endpoint found in the enterprise and partner WSDL files. For the longest time, this endpoint was https://www.salesforce.com. Last year, we quietly launched a new endpoint: https://login.salesforce.com and in a future release this will become the official endpoint listed in the WSDL files. The old endpoint will continue to work, but you will shave off a bit of request time if you switch to the new endpoint, so why not do it right away?
Note that this change only impacts the login endpoint. As always, once you have logged in, you will receive the organization specific endpoint in the SOAP response and these endpoints have not changed.