The issue over whether Google is giving preferential treatment to its own products is back in the news, and this time the evidence is pretty damning.

I’m not sure how Google will tap dance around it. (See Trevor’s reporting about another Google+ tapdance here.) Hopefully, they won’t even try. They’ll just come clean and admit the rules have changed.

The new shoe that’s dropped: Engineers from Facebook, MySpace and Twitter created a “Don’t be evil” toolbar that allows users to see what results would have been before Google started the Search Plus Your World product as the default for users. Rather than getting into what should make a profile more relevant, the engineers just show what Google’s own indexed results would have produced before the change. The master of search, Danny Sullivan, has a thorough walk through of the product here and John Battelle has a nice story of how it came about here.

Every reporter loves a big nerd fight– and make no mistake, this is one. Although it’s something individuals at these companies did, not the companies themselves, you’d have to believe in Santa Claus and the Tooth Fairy not to think competitive dynamics aren’t a big part of this.

But ultimately, none of that matters. This isn’t an issue of whether Google– as a for profit company– should have the right to push its own services over others. This isn’t an issue over what Twitter or Facebook would do in Google’s shoes. To me, the interesting issue isn’t even anti-trust. This is an issue of what Google promised users back when it went public, without a gun to its head, without any pressure from any competitors to do the right thing.

This is a Google v. Google issue.

Here’s what Larry Page said in his Playboy interview before the company went public: “Most portals show their own content above content elsewhere on the web. We feel that’s a conflict of interest, analogous to taking money for search results. Their search engine doesn’t necessarily provide the best results; it provides the portal’s results. Google conscientiously tries to stay away from that. We want to get you out of Google and to the right place as fast as possible. It’s a very different model.”

Here’s what the site says under “Ten Things We Know to Be True:” “Advertising on Google is always clearly identified as a ‘Sponsored Link,’ so it does not compromise the integrity of our search results. We never manipulate rankings to put our partners higher in our search results and no one can buy better PageRank. Our users trust our objectivity and no short-term gain could ever justify breaching that trust.”

Here’s what the S-1 said when Google filed to go public, “Google users trust our systems to help them with important decisions: medical, financial and many others. Our search results are the best we know how to produce. They are unbiased and objective, and we do not accept payment for them or for inclusion or more frequent updating. We also display advertising, which we work hard to make relevant, and we label it clearly. This is similar to a well-run newspaper, where the advertisements are clear and the articles are not influenced by the advertisers’ payments. We believe it is important for everyone to have access to the best information and research, not only to the information people pay for you to see.”

And here’s what Eric Schmidt told Congress, when asked whether Google ever “cooked” results: “Senator, I can assure you we have not cooked anything.”