Although I understand Google's stance on this, I w...

Although I understand Google's stance on this, I would recommend not referring to it as a "service" when Google is not doing anything.

I propose a modification of this concept. Google already tracks clicks on its search results, so why not take this a step further? Each time a user clicks a search result, Google POSTs the user's IP address to the page in question, which would then allow that IP to view full pages for the next X minutes (something short, 5-10). After that point, that IP would be shown the usual registration page for Y amount of time (from 1-24 hours). In this fashion, the user could be verified as having visited from Google results (only allow the POST from Google IPs), and it would provide users with full results for a short time (enough to answer a simple question, since sometimes the first page does not have a suitable answer) but not cause a site's premium content to be freely accessible at all times using simple referrer spoofing, which could easily be automated via a small addon in any of the major browsers.

In addition, this would indeed actually be a Google service, since Google would be providing a form of authentication.