In the estimation of the community around here, what other technologies mite work not only for low-grade missile defense, but also for modern ICBMs?

articles in Janes, Milspec etc have suggested (and I see no reason not to agree) that future Nuke delivery systems from advanced nations (China, Russia) will be Stealthy- Stealth cruise missiles, stealth bombers, super-cavitating super-sonic torpedoes, sea-launched fast medium ranged Missiles and cruise missiles, etc. How will missile defense systems do anything to defend against these systems? Will they only encourage nations to invest in the creation of these systems?

Also, it doesn't take much to see that the best way to nuke america 'on the cheap' is a nuke in a shipping container full of scrap metal parts mixed with lead. (very hard to detect). Would the development of missile defenses do anything to discourage this (already quite viable) approach?

Davin