I can't be the first person to have this issue but I can't seem to search out a solution. I have a large box that I'm building in. About 100x100x20. I have glowstone all around the perimeter and I also have the room grided out so I have lines of glowstone every 10 blocks for the length and width of floor. I also have glowstone on the walls at 10x intervals at the 7 and 17 mark up the 20x wall. Suffice to say I would think this is a lit space.
I still get spawns, and not only do I get spawns but if I am across the room up high looking down the ground is in shadows... not dark just darker that I would expect. If I move over to the location it typically takes a couple of seconds for the 'light' to pop and brighten up the area.
I don't have a slow computer but is this just lag or something else? Its pretty consistent if I am some place in the room the lights dim in the farthest location away.
EDIT: Version 1.2.5
Don't know if it factors into things but the building splits 3 biomes. It's a flat creation but I have desert, grass and snow. Actually there is a diagonal swatch of snow that cuts thru the building from corner to corner. I can have desert , snow storms and then grass and rain all within a couple of steps.