
The Mouse System Preferences pane on Leopard does not include an option to enable momentum scrolling like Snow Leopard does. However, by setting a hidden preference, you can get this feature without updating to Snow Leopard.
To enable momentum scrolling, first run the following command in Terminal:
defaults write com.apple.driver.AppleBluetoothMultitouch.mouse MouseMomentumScroll -bool yes
Next, go to the Mouse System Preferences panel, and uncheck and then check the Scroll option. You should now have momentum scrolling enabled.
[
robg adds: I don't have a Magic Mouse, so I can't test this one.]