Adding to this whole MAD Idea, you should probably be allowerd to build a "public fallout shelter" improvement in cities. You could remove the 50/50 chance of each citizen surviving in a nuked city, UNLESS it has a shelter in it. If you include that the city is completley destroyed, the survivors could emmerge from the shelter as settlers/workers a few turns after the blast. This would allow for some interesting post-apocalyptic scenarios.
But plannet busters? A definate no. Unless you want your Civ4 Games to end up like this:
Shaka: Ah Mr. Bond, welcome to my underground lair.
Bond: You;ll never ger away with this Shaka!
Shaka: That's what you think Mr. Bond! You see, I have constructed a highly advanced nukelear device which I call a, "Plannet Buster." This,, "Plannet Buster," posesses an Iron fusion core that gives it enough destructive force to: LEVEL ENTIRE CONTINENTS!!!!!

DUN DUN DUN!!!!
Bond: Your a madman Shaka!
Shaka: Scilence! Impi, attach our guest to the device.
Ok, now put me through to the UN!!!
Alexander: What do you want now Shaka?
Shaka: I have constructed a highly advanced nukelear device which I call a, "Plannet Buster." This,, "Plannet Buster," posesses an Iron fusion core that gives it enough destructive force to: LEVEL ENTIRE CONTINENTS!!!!!

I will spare you the wrath of this "Plannet Buster" if you pay us, One Hundred Billion GPT!!!!
DUN DUN DUN!!!!
Alexander: But that's impossible!
Shaka: You have 5 turns gentlemen. Shaka out.