A suggestion, but I'm placing it here because it'd pretty much be needed only in Survival.
I say "sponge," but that's really only because a sponge block is anti-water. I think that a block that is anit-spawn would be fantastically useful.
But notice that I don't say anti-mob. This wouldn't be a safe area, not like some sort of "magic circle," but just a block that within the radius mobs can't spawn.
I'm not 100% familiar on the rules of spawning; I don't think anyone other than Notch is. But I've seen quite a few unfortunate spawnings resulting in my quick and "unfair" death, and a number of complaints on how no place is "safe."
The thought on this is that it'd allow bases and hideouts to be "spawn-free." While mobs could still find the place and kill you if you're careless or stupid, the location would be a good place for a break.
Natually, since Survival is supposed to be really hard, this shouldn't be a nerfed thing. I'd say limit the effectiveness to a 10-block radius, just enough for a small base, and limit one block per player. Once crafting is done, it should be incredibly difficult and/or expensive to make/buy/whatever, so only the best players, or a group of good players, can expand their bases at a great cost. In addition, maybe make it so that any kills made while you are in the "safe" area don't award you any points, so there's no benefit in building the perfect base just to rack up points.
On IRC Notch mentioned thinking about adding a flag block that marked a base. Mobs wouldn't die of loneliness there so you could make a pig farm and a couple other things I forget. I think it could also work to stop mobs from spawning.
it would be terrible if it was a mating thing. you could kill all of the creepers in one area which means no more creepers there. move onto next part same thing happens but if they spawn randomly their kind will always live for better or worse.