It looks like SD spots on an HD channel have full, or at least 95% full raster. This was my scientific evaluation looking closely at where titles and text were placed in the spots 🙂
Also, regular SD channels are uprezed by my Motorola digital cable HD box to 1080i, and it appears it also put the 4:3 at full raster (with side letterbox).
So action safe really isn’t so safe anymore.