While speaking at an event for the Lunar X-Prize, Brin said that Microsoft's bid for Yahoo is "unnerving" -- though I doubt Google really has anything to worry about in the short term. Many people think this is a simple case of the pot calling the kettle black.
"The Internet has evolved from open standards, having a diversity of companies, and when you start to have companies that control the operating system, control the browsers, they really tie up the top Web sites, and can be used to manipulate stuff in various ways. I think that's unnerving." -- Sergey Brin
On Google's official blog, David Drummond went on record opposing the merger with strong words shortly after Microsoft made the announcement.
"Could Microsoft now attempt to exert the same sort of inappropriate and illegal influence over the Internet that it did with the PC? While the Internet rewards competitive innovation, Microsoft has frequently sought to establish proprietary monopolies -- and then leverage its dominance into new, adjacent markets." -- David Drummond
I suspect these worries will not prevent the deal from going ahead -- but it might be enough to plant seeds of doubt in regulators minds. If successful, this could really slow down the inevitable acquisition process.