Platform: Apple iPhone, Apple iPod touch
The map is static (you can't zoom in) and on the sluggish side to respond, but fighting and moving your troops around is easily done. What's less rewarding are the cards and missions: you can't see what cards you win or are holding, and only real Risk-heads will have all the mission bonuses memorised.
There are pass-and-play and Wi-Fi multiplayer options but no online battling - a wise decision as nothing would be more boring than waiting for someone in Kyoto to finish up a long game. The computer AIs are pretty good, although let's be honest, Risk's military tactics aren't exactly Sandhurst difficult: conquer South America and Australia early on, then just sit there until everyone else is dead.