I got the same question during a presentation I was giving recently and have to admit I didn't knew the answer on the spot either. Some thinking and discussing with others has given me a pretty clear picture on this though, which I will now try to pass on to you.
I don't think code order matters that much anymore. A couple years ago, when Google was crawling only a portion of a large webpage (mostly due to hardware restrictions), you'd better make sure your valuable text or links were placed in the first part of your webpage, otherwise it wouldn't get crawled at all! With Google crawling large webpage in total (if I recall some quote from Matt Cutts correctly he stated that they now index webpages over several MBs in size, as long as they contain enough valuable information).
With Google advancing their detection of the 'visual location' where text and links are placed on a webpage (see #5 on http://www.seomoz.org/blog/10-illustrations-on-search-engines-valuation-of-links), source code ordering will most likely have dropped in value as well. Using CSS styling, we can now order our source code pretty much at will anyway, which has changed it from a valid signal to a 'SEO trick' (just like adding a suffix to the URL has, see http://www.finishjoomla.com/blog/5/does-adding-a-suffix-to-my-urls-affect-my-seo/).
By 'viewing' (and perhaps manually categorizing or using machine learning http://en.wikipedia.org/wiki/Machine_learning) webpages Google will notice patterns in webpage source code. For example: 'that div containing a large amount of links, usually placed in an ul-li, often containing links to 'home' and 'contact' will most likely be your menu. Just like 'that div containing more text than any other div, often starting with a H1 or H2 tag, containing the most images and ending with a call-to-action' link will most likely be your page content area. Thus, Google doesn't 'know' whether a certain part of your source code is your menu, your sidebar or your page content, it deducts it by looking at common patterns.
(lol, my answer is more than three times as long as your short question!)