You are probably right that most analytics packages will fail without the referrer headers which brings me to worry more once browsers supports HTML5 and its rel=”noreferrer” tag.

Edit: Just remembered having an issue with this. It only work in modifying the *first* visit that came directly from the search engine since I was relying on the “Referral” field. This meant that subsequent visits (e.g. from a page refresh after 30 minutes) will be tracked inaccurately. This will still work fine if the purpose is just to understand entrances. Otherwise, it may be better off using _addOrganic() method.

However, while this system will seemingly track Google regional engines well, what about Bing?

Bing doesn't use variant domains for different regions; worse, they don't appear to use query strings identifying regions that can be parsed. Perhaps this is a limitation of Google Analytics when it comes to Bing (and many other situations in GA, such as redirects where referrer-identifying parameters are dropped in the final rendered URL that GA uses). Perhaps is a hard limitation of all analytics packages: it would be interesting to know if enterprise packages like Omniture or Coremetrics displays regional Bing information.

A bit of red herring, I know, as Bing barely registers compared to Google in most non-US markets. But this may become more of an issue if Bing gains more international market share.