Apple released the Magic Trackpad this morning, but the OS update to make it work with your Mac is out now, available via Software Update on your Mac (drop down the Apple menu on the left if you've never done this before). Here's the link for a download of the update.
This update also enables three-finger drag gesture and inertial scrolling for older Mac portables listed on this support page for the update. Something our own Chris Rawson pointed out was possible.
Oh, and there's a new Preference pane! From Apple's page:
This update also installs a Trackpad System Preferences pane for configuring Magic Trackpad on Intel-based Mac desktop computers that support Mac OS X v10.6.4 and above.
If you find anything wonky, let us know in the comments (and report the bug to Apple as well).