ICBM is available with Rocketry (plus others). Rocketry leads to Satellites. SDI is available with Satellites. I never checked prepatch but it seems SDI becomes available AFTER nukes. ICBM has a cost of 500. SDI has a cost of 500 (double IF you have plutonium). SDI pretty much makes bomb shelters not worth building. I've never had the AI launch a nuke at me, but destroying tons of tile improvements, crippling my city's population, damaging/destroying my units nearby just doesn't sound like something i want happening. I'm thinking SDI is more in place for mutliplayer rather than single player as even 75% of nukes landing could ruin the fun quickly, since nukes are very overpowered (well they are nukes).