Interesting video directly from the source. Video 7 Tires and Wheels.
http://www.bmwusa.com/Standard/Conte...tionFlash.aspx
It states that you must have at least 1/8" of tread left and if the car came with runflats on it when you leased it, it must be returned with runflats. So if your spec sheet on your lease states non-runflats you are good.