It doesn't poll at all. The RIM API includes a listener that will call the program whenever the backlight status changes. The delay option is how long to wait AFTER the backlight goes off. So if you set your backlight timeout to 30 seconds, and then the delay in AutoLock to 30 seconds, it'll be a minute before it locks the keys. For this, it just waits the additional delay, and checks again, doesn't poll either.
I can't see the white screen thing happening, but that doesn't mean it's not. Just means my eyes aren't fast enough.
New planned feature: Option to not lock if the device is plugged in to a power source.