Interesting... at the same time as Magic mouse comes into stock (well, in UK Apple stores, anyhow), Apple release the "Wireless Mouse Software Update 1.0 for Leopard" & "Wireless Mouse Software Update 1.0 for Snow Leopard".
Here are direct links to the installer disk images, to save you time - even if you don't yet have a Magic mouse, you can install the software now to save time later:
Leopard:
http://support.apple.com/downloads/DL951/en_US/WirelessMouseSoftware.dmg
Snow Leopard:
http://support.apple.com/downloads/DL950/en_US/WirelessMouseSoftware.dmg
I also found it interesting, how the mouse icon has changed into a Magic mouse in System Preferences:

Here are direct links to the installer disk images, to save you time - even if you don't yet have a Magic mouse, you can install the software now to save time later:
Leopard:
http://support.apple.com/downloads/DL951/en_US/WirelessMouseSoftware.dmg
Snow Leopard:
http://support.apple.com/downloads/DL950/en_US/WirelessMouseSoftware.dmg
I also found it interesting, how the mouse icon has changed into a Magic mouse in System Preferences:

