Definitely cannot imagine an accelerometer. If the mockups are correct, the screen will be similar in width to the 1st/2nd gen Nanos...
not sure what mockups you are seeing, but the one i tend to believe shows a screen of similar width to the current Nano, but much taller so you can tilt it to the side and watch movies in widescreen ratios. it will be the biggest screen on an iPod to date, minus the Touch and iPhone of course.