Just to clarify, Jeff is still using POST to send the XML across. It just seems some servers have overly aggressive security settings which wouldn't allow either raw posts, or allow PHP to read directly from the input buffer used (I'm not sure I agree with the perceived security advantage). But it's best for you to work with your host to establish a workaround.
Also, since it's 403 errors, the Apache server logs should have some sort of error message, which will go a long way in helping to understand what's happening behind the scenes. Your host should be able to provide that.