I bought my Magic Mouse yesterday at the local Apple Store.  I did everything according to the instructions:

• Download and install the WirelessMouseSoftware update for Snow Leopard. Check.
• Pair the Bluetooth mouse with my Mac Pro. Check.
• Go to the Mouse System Preference to see the new options. Check… wait a minute…

What?! No gesture options showed up!

After some quick online problem research, I saw that USB Overdrive — a utility I’ve used in the past — could be the source of a conflict. I ran the downloaded and ran USB Uninstaller. It reported that USB Overdrive was not currently installed, therefore, there was nothing to un-install.

Re-downloaded the software, restarted, retried. Still, no gesture options.

After much frustration and much trial and error, I came upon the solution. Although the USB Overdrive uninstaller didn’t detect it, there was indeed a file called USBOverdrive.kext in System/Library/Extensions. I tossed it out — restarted — and bingo! Magic Mouse gestures now available!

Problem solved.