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:
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.