I'm pretty sure everybody realizes this now, but I'd like to reiterate: The Official Google Blog isn't a blog. It's a press release and public relations machine that happens to use a blog platform.
The Google Blog has been changing over the years, as Donna Bogatin notes. But the change isn't mysterious--the company has simply grown up into a bigger, more professional version of its inherently secretive self. The blog is used less for insight into Google and more to seed the blogosphere--which is only too happy to link to it--with pro-Google news. And has anyone noticed each Google blog has a similar "voice." It's almost like--no, couldn't be--the posts were approved by marketing personnel.
Anyway, this isn't nefarious. It's just good PR policy.