Hold on a second...
Would it be surprising that google's own services appear on the top of their search functions even if the algorithms used to dictate the page rank were completely honest?
By which I mean: when people use google searches, aren't they more likely to use google services?
Here's my example that I'm guilty of. I'll put a street address into a google search, let it autocorrect it and immediately hit "maps" up at the top to view the address in google maps. I'm not actually clicking on the search result, though?
The thing here that's essentially an issue is that finding the answer to a query in google is almost always going to require less clicks/page loads/typing than using another search engine. The integration of their services is something that other sites genuinely can't battle against more than the pagerank stuff, I think...