If you’re going to use an apache proxy to fix some of the issues with XMLHttpRequest, be aware that mod_alias sends a redirect to the browser. That is, it sends one of the 3XX HTTP status codes to the XMLHttpRequest object. The XMLHttpRequest object then issues the GET itself (well, some do, check out these XMLHttpRequest tests for more). That’s fine if you’re doing a GET, but if you’re doing a POST, then some redirects will require user interaction. I found that mod_alias, which was sending a 301 (redirect permanent) just turned the POST request into a GET. (mod_alias doesn’t always come to the rescue, apparently.)
The solution? Well, since you are really proxying the XMLHttpRequest’s request, use mod_proxy.