I just now heard about it via a brief article in The Scientist (“Google Scholar welcomed: New academic search service is applauded but librarians, academics have some reservations”—by Doug Payne) at this URL:

<snip>However, Michael Eisen, co-founder of another open-access publisher, Public Library of Science (PLoS), pointed out that people using the service will face an important obstacle—many of the articles are only available to journal subscribers.
“While Google may have the noble goal of making ‘the world’s scientific literature universally accessible,’ the fact remains that most of the articles returned in a Google Scholar search can only be accessed by those fortunate enough to have a subscription to the relevant journal,” he told The Scientist.
A PloS spokeswoman added: “Google will find that they can better serve their searchers’ needs for access to complete scholarly articles by ‘flagging’ as open access or ranking more highly those that are freely available online. Such a system would minimize people’s frustration at finding an article that looks perfect for their research needs but discovering that they are unable to access it. This frustration is already quite evident in the various threaded discussions occurring online.”<snip>