Here, define it for yourself. :)
http://jsfiddle.net/dJ57T/3/

Safari:
Focuses on adding to DOM.

Chrome:
Focuses on adding to DOM if it's the first input with autofocus. First as in never has there been an autofocus input before in the DOM before.

Firefox:
Never focuses (neither on document load neither on adding to DOM), but that might be a JSFiddle thing. [Quirksmode](http://www.quirksmode.org/html5/tests/inputs.html) says they support it and since MDN was my source of learning about it (quotes above :)) it's probably true.

MDN:
> This Boolean attribute lets you specify that a form control should have input focus when the page loads, unless the user overrides it, for example by typing in a different control. Only one form element in a document can have the autofocus attribute, which is a Boolean. 

W3C wiki:
> Another common feature that previously had to rely on scripting is having a form field automatically focused when a page is loaded. This can now be achieved with the autofocus attribute … Keep in mind that you shouldn't have more than one autofocus form control on a single page.

Conclusion: don't rely on it and so that we're within the definition of having one thing with autofocus, I wouldn't apply it 'just in case' either. .focus() does its job (well, sometimes with the help of a timeout :)), so let's stick to that.