From AppleInsider:
"Apple posts Bluetooth Firmware Update 2.0
A lower-key update has come Wednesday evening in the form of Bluetooth Firmware Update 2.0 (1.8MB).
Upgrading provides various bug fixes but is targeted at compatibility between certain Macs and both the Wireless Mighty Mouse as well as the Wireless Keyboard.
Only Macs with a Broadcom-made Bluetooth chipset will see the update. Installing requires Mac OS X 10.5.7 or later."
ON EDIT: I'm running 10.5.7, so I must not have the Broadcom-made BT chipset. I'm on a late 2007 24" 2.4 GHz Al iMac.