In the 1.5 prerelease, if a world is created with a ceiling 100 blocks from the ground, and a hole is made and then sealed in that ceiling, the ground will believe it is in range of sunlight and will be light level 16. Moving around lights up more areas, and placing/breaking blocks does the same. This can be reproduced using a superflat world and the preset "2;7,10x1,100x0,7;1".