Google is upgrading its search infrastructure and it’s being really shady about it.

In a post on its Webmaster Central blog, however, Google engineers Sitaram Iyer and Matt Cutts insist that ordinary users won’t even see the difference.

“For the last several months, a large team of Googlers has been working on a secret project: a next-generation architecture for Google’s web search,” the post reads, making it all sound vaguely like some kind of elf workshop. “It’s the first step in a process that will let us push the envelope on size, indexing speed, accuracy, comprehensiveness and other dimensions.” The user interface is unchanged.

Developers are encouraged to try out the new technology on a “sandbox” page and then offer feedback by including the word “caffeine” in Google’s feedback text field, secret-password-style.

The company acknowledged that “some parts of this system aren’t completely finished yet.” But the industry buzz is obviously a huge part of it: There’s a legitimate new contender in the search engine market, Microsoft’s Bing, which is fueled by heavy marketing dollars and has begun to inch its way up in market share since its debut earlier this summer.

Google CEO Eric Schmidt gives the impression that he isn’t particularly worried about Bing. But it’s hard to not look at a shadowy blog post about under-the-radar upgrades to Google’s search index and not take it as a Googly way of saying, “game on.”