Glad to hear I'm not the only one fooling around with PW's in shops..... It was actually pretty dark under my jacket, I just felt like an idiot doing it! But yes, I do appreciate it's not quite the same.
Incidentally, I have heard of a few people buying Kobo Glo's who have asked to take the one they've just bought into a cupboard in the store so they can check it in the dark for light spots. But the key difference is they aren't just testing them, they have bought them first and are just checking quickly for an obvious defect so they can exchange it there and then.