The W3C announced that they would standardize the de-facto XHR object and the first draft has been released.

What does this mean for us? Well, it gives you some documentation, has a few hints from editors on curent implementation notes, and let's us know that new XMLHttpRequest() will be the way to do things in the future (as it almost is with IE7 coming on board etc).
Example

// The following script: var r = new XMLHttpRequest(); r.open('get', 'demo.cgi'); r.setRequestHeader('X-Test', 'one'); r.setRequestHeader('X-Test', 'two'); r.send(null); // ...would result in the following header being sent: ... X-Test: one, two ...

[Source: ajaxian.com]