Okay, I see the roll-back behaviour. Doesn't seem quite the same as what distracted me in the first place - but maybe it's a matter of degree. I guess this is a minor bug in the way the light sensor behaves when peeling back the smart cover.
Didn't touch it to make it brighter; it stayed dim - I think actually faded a little - and then the screen locked, after maybe 20 seconds.
I didn't open the cover especially slowly, I don't think. This doesn't seem to be just light sensor adjusting behaviour.
Is it the iPad acting as if the cover went straight back on? [edit] No, playing around with the magnets suggests that in that case it switches off again immediately. The behaviour is more like what happens when iPad reaches an autolock timeout: short period of very dim screen, then lock.
So it seems the bug is that sometimes the iPad wakes from sleep, via the smart cover, and immediately thinks it's time to prepare to autolock. Because it was put to sleep just ahead of its normal timeout period?
Yeah, not too worried. It's a niggle though, and Apple will only fix it if they know about it, and if someone reports it having figured out what exactly the bug is...