It's true. It's never brought up as a matter of principle. In fact, a ranking change that could theoretically reward a site for using Google products would not launch.
Didn't AdWords advertisers get a quality score discount (through higher ad clickthrough rates + free checkout processing) for using Google Checkout?
The official page for Google Checkout to this day states "Google Checkout users click on ads 10% more when the ad displays the Checkout badge, meaning more traffic to your site."
That is a pretty clear-cut example of Google tilting their "algorithm" toward promoting another Google product. And in an area where one of our small clients didn't use Google Checkout, that change simply priced him out of the ad auction. His profit margins after ad costs were roughly 10% & with that 10% cut in relative clickthrough rate he no longer had a place in Google's ad auction. He was forced to use another Google product if he wanted to be profitable with AdWords.
Of course one could say that the organic results are different than the paid results, but the Google editions ebooks ranked quickly in the US after the ebook store was launched, Google only rolled out universal search after they bought YouTube, maps & local results now come with tags that earn incremental revenues from the "organic" search results, etc.
