A reader pointed us to fact that Google appears to have upped the number of ads it displays atop its search results. Sure enough, Google confirms that for highly commerical searches, it’s now going to display as many as three ads atop its search page, up from two before. The change took effect in the last week. (We’ve seen it intermittently.)

Google is pulling up an extra high-performing ad from its ad stack on the side of search results and moving it to the more promiment top-center placement. Google’s argument is that, for commercial searches, such as “DVD player,” ads are often as relevant to searchers as the regular search results.

“Google is always working on improving the user experience and ad effectiveness,” spokesman Mike Mayzel told us. “For highly commercial queries, we believe an additional ad above the search results is good for both.”

Yahoo limits the number of ads above search results to four, although two or three ads is more common.

It’s a small tweak for Google, to be sure. But noteworthy given Google’s famously uncluttered user interface and its commitment to usability. Will users notice the change?