Apple has released two updates that enable users running either Leopard [1] or Snow Leopard [2] to take advantage of the multi-touch capabilities in the new Magic Mouse (see "Apple Releases Magic Mouse, New Remote, Souped-Up Base Stations [3]," 20 October 2009). Apple notes that users running Leopard must have version 10.5.8 installed, and that Momentum scrolling is not available. Users running Snow Leopard must have version 10.6.1 installed. Both updates are available via Software Update or the Apple Support Downloads page. (Free, 36.22/63.92 MB)
[1]: http://support.apple.com/kb/DL951
[2]: http://support.apple.com/kb/DL950
[3]: http://db.tidbits.com/article/10663