Officially, Apple doesn't support momentum scrolling on their new Bluetooth Magic Mouse under Leopard:
However, you can enable it using an (in)famous defaults write property. Open your Terminal console and type:
defaults write com.apple.driver.AppleBluetoothMultitouch.mouse MouseMomentumScroll -bool true
Disconnect and reconnect your wireless magic mouse, and you will get momentum scrolling.
To disable the property, simply set to false the propery above.