According to Minepedia/Light, the changes that occurred in the Halloween update make torches useless for level 16 and below (incidentally, also the level where diamonds start).
Now, that’s all fine and dandy, but if a Mob such as a Creeper spawns right next to the player, the player has no time to react. This is particularly bad with Creepers, because one spawning next to you is generally impossible to avoid, particularly in tunnels and such.
That doesn’t seem like it would be fair at all (to make next-to-random insta-deaths-by-creeper for the player), and I would be surprised if it behaved that way, but I’ve not heard or seen anything that indicates that it does not operate that way.
More to the point, even in dark areas I’ve never had a mob spawn right in front of my face. Sometimes if I’m on top of a hill or something of that sort you can see one spawn in the distance, but never next to you. This leads me to believe that it can’t happen, though again I’ve had no confirmation of this.
Can a mob spawn right next to you in Minecraft?
N.B. I’m referring to darkness-induced spawning. Of course a mob spawner can cause mobs that spawn right next to you 😉
Unless other changes have been made to the spawning code, enemy mobs can’t spawn within 24 blocks of your current location. The exception to this is of course mob spawners, but that’s a different story.