Are you across the room during the "period of non-use," or are you at the computer? Any chance you're between a light source and the MBP's ambient light sensor, thereby shading it, and that you unconsciously shift position, thereby unshading it?
Thanks for the thought, but that's not my issue, which is that even in a pitch black room with my pref set for the backlight to never go out it goes out anyhow.
The light sensors are located in your computer’s speaker grilles, so make sure the grilles are not covered when you use your computer in low light conditions.
Again, though, that's got nothing to do with what's going on; the backlight goes on when it's supposed to go on, but it doesn't stay on as per my pref.
I didn't go thorough every Google hit on the subject, but there appear to be, i.e. have been, any number of assorted backlight issues, none of them resolved; I even found one poster who was told by AppleCare to reinstall OS X, and even that didn't work. (I think I'd rather live with the annoyance than reinstall OS X again.)
I've got to go to the Genius Bar anyhow; maybe I'll get lucky there.