Running Moodle 2 with any theme causes layout to break when page first loads. Moving mouse over elements on the page or to the corners of the site fixes this. Applies to any theme running in "Modern browser" mode.
To reproduce, install Moodle Moodle 2.0 dev (Build: 20100430)
as normal. Bug should be evident without requiring any changes to site settings, core code etc.
Haven't tested with any later Moodle builds nor with other browsers, so it may or may not have been fixed in the mean time.
See URL for video of bug in action