Skeletons and Zombies do not burn in daylight while standing on slabs, stone or otherwise. If they begin to burn, they will run for cover on TOP of slabs that are otherwise in the open during the daylight and will stop burning while on them.

To replicate, put some slabs down on the ground, make some Zombies and/or Skeletons next to it in the daylight, then watch their behavior.

Similarly, and although i cannot confirm this, i don't think mobs are spawning on slabs either. I don't know if that's intentional or not. I have a tunnel that is lined with stone slabs and, although it isn't lit, nothing spawns down there.