iSamurai is an accelerometer-based iPhone game that recognizes certain gestures as specific sword fighting attacks or blocks. The developer's video shows it well:
The game comes with both two player (WiFi) modes and single player modes. The single player mode is surprisingly well implemented and pits you up against a computer opponent who calls out their attacks (like Left strike or High strike). In this mode, you must defend yourself by performing the correct block and finding your spot to attack. Meanwhile, two-player mode acts as you'd expect, allowing you to trade blows with a real life opponent.
The moves take a little practice to learn but the game comes with an extensive tutorial with videos of the exact movements.
App Store Link: iSamurai, $0.99