The new Apple Magic Mouse has been available on the Apple Store and on Amazon since a couple days after it was announced on October 20th. MacRumors reports that the first Magic Mouse buyers already got shipping notices from Apple.
On Amazon the Magic Mouse is still only available as pre-order without a shipping date, but the Magic Mouse is already under the top 10 bestselling products in the Electronics category. The Magic Mouse is in the 8th place outselling the Flip.
It looks like Apple has a hot selling accessory on their hands with the new Magic Mouse.
You need to install OS X Updates to run the Magic Mouse. The upcoming OS X 10.6.2 will most likely have the Magic Mouse drivers integrated.