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
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091104090207286