Steven Sinofsky in a post about the activity regarding Windows 7 and batteries, specifically the new Windows 7 message “Considering replacing your battery” explains why the alerts surprised some users: The messages result from a new feature in Windows 7 that alerts users when their batteries are performing at 40 percent or less of designed capacity. "Essentially the battery was degrading but it was not evident to the customer until Windows 7 made this information available," Sinofsky writes in the post. "We recognize that this has the appearance of Windows 7 'causing' the change in performance, but in reality all Windows 7 did was report what was already the case." In other words, someone who upgraded their machine to Windows 7 from Windows Vista or Windows XP wouldn't have previously seen the alert. The Windows chief gives extensive background and context.
More info: Windows 7 Battery Notification Messages