Then there's Jericho the series and Mad Max 2 which doesn't have the exchange but the aftermath.

Rhapsody in August by Akira Kurosawa was a kind of look at life after the Nagasaki atomic bombing during WW II.

Final War about America nuclear bombing South Korea these are old movies though not huge hollywood blockbusters.

It won't be a build up to nuclear war movie, but rather a post apocalyptic survival movie. Look for "The Road" based on Cormac McCarthy's Pulitzer prize winning novel of the same name. You are never told in the book specifically that there was a nuclear war, but the main characters are obviously surviving a long nuclear winter.

It is scheduled for release in 11/08 at this point. The star will be Viggo Mortensen. I have very high hopes for this movie as the book was one of the most powerful and haunting I have ever read. I pray it is made mostly in black and white, as there are only very brief glimpses of color in the ashen gray landscape the main characters (a father and son) trudge through looking for any safe haven. The short color scenes are literally rays of sunshine indicating hope.

I can't recommend the book more highly, you will be a better person for reading it, IMHO.