There's actually an existing, playable, Fallout mod for Hearts of Iron II: Armageddon. It covers an amazing variety of factions as separate nations across the entire North American continent, from Mexico to Canada. The beta is now in 0.3, and let me tell you, the team working on this stuff is GOOD. If you own HOI with the Armageddon expansion, do NOT miss this amazing mod.

http://www.gamesas.com/index.php?act=findpost&pid=0

EDIT: Need I mention it's got working multiplayer?

Absolutely. There's a lot of very dedicated, talented people working on this, and it really shows. There are a wide variety of events and countries from past Fallout games and entirely new ones designed in the spirit of Fallout. Who hasn't wanted to play NCR and take on the Enclave...or the remnants of the USA and restore your rightful rule across the continent?