Update: It seems like this was caused by a Chrome extension named FastestChrome. So Google is not testing infinite scroll in the web results.
Matt Beck, a reader, emailed me a video of Google testing a form of infinite scroll in the search results. Infinite scroll is when you do not have to click the next page button on the search results to see additional search results. Instead, scrolling down will automatically load more search results.
Google does this with their new image search and now appears to be testing it in the web results.
Here is that video from Matt Beck:
Notice the pagination is automatic. When you scroll to the bottom of page one, there is this black bar that moves you to page two automatically. Here is a picture: