Enable Momentum Scrolling for Magic Mouse in 10.5
If you happen to have a new Magic Mouse running on OS X 10.5, you may be disappointed to find that it lacks a feature you get with Snow Leopard: momentum scrolling. Momentum scrolling is the iPhone-like behavior whereby you can flick your finger and have a list continue to scroll while slowly coming to a stop. In 10.6, you can enable this option for your Magic Mouse, but the preference is missing in 10.5.
However, thanks to Mac OS X Hints contributor dougbarth, you can now add momentum scrolling to your Magic Mouse in 10.5. Launch Terminal (in Applications -> Utilities), paste this command, and press Return:
defaults write com.apple.driver.AppleBluetoothMultitouch.mouse MouseMomentumScroll -bool yes
Next, open the Mouse System Preferences panel, and look at the One Finger section of the panel. Disable (uncheck) the Scroll box, and you should be good to go--flick your finger down the Magic Mouse's surface, and whatever document is frontmost should scroll, then continue scrolling while slowly coming to a stop.
Note that I don't have a Magic Mouse of my own to test this with, but readers report that it does work, at least in OS X 10.5.8. If you tire of this behavior, you can just repeat the above command, but replace yes with no.