I want that and I don't even have Minecraft yet (the money should arrive tomorrow) so I went and downloaded
Clonk Endeavour instead which is a kinda similar game that I played back in '99 (well, I played Clonk 4 but Endeavour is just a more developed version of the same codebase).
From what I read on the wiki a redstone torch's light is just below the threshold that prevents monster spawning, while the chance goes down with more light it's still not impossible for monsters to spawn right under the torch and then there's falloff to account for.