Well I'm using the Magic Mouse with Lion and I gotta say, it works just fine for Lion!
In Safari, one finger swipe left or right to go back and fourth, two finger swipe left or right to switch between spaces/full-screen apps, double tap with two fingers to launch mission control, and one finger tap within certain apps such as Safari to zoom in the page.
Honestly, with the exception of no gesture for launching Launchpad with the mouse, I don't see why I'd need a magic trackpad when I already own a Magic Mouse.