That's funny they release the first software update before the hardware is available in stores. I undestand 27" iMac, though.
I have the feeling that in today's industry nobody can wait: why, simply, do not launch an item when ready, instead of pre-launch in weeks before, to figure out that it needs an upgrade, and so on?
This isn't an update to the capabilities of the magic mouse. This is an update to the OS so that it can work with the mouse.