Swiping to switch spaces stopped working in Lion
A couple days ago I noticed that I could no longer switch spaces using a three-finger swipe gesture on my magic trackpad. I finally got it working again and thought I'd share in case someone else is having this problem:
Make sure 'Swipe between full-screen apps' is enabled in the System Preferences --> Trackpad --> More Gestures. (It should already be on, of course.)
Take Safari full screen by clicking the diagonal arrows doodad in the top right corner of the window.
That's it. Once I took Safari full screen, swiping between spaces started working again (and continued working after Safari was no longer full screen).
Hope this is useful to someone.