Macrumors forum member michaelb, in a fit of jealousy over his ladyfriend's new unibody MacBook, decided to try to trick his last-gen MacBook Air into recognising four-finger gestures. He installed the updated 10.5.5 system from the new MacBook onto his Air, and then through some tricky manipulation of the kernel extension, managed to get the flashy four-finger gestures working. This is like the invention of sliced bread times infinity.
He can use Exposé and Application Switcher just like the newer models, and suspects all later-model MacBooks could also work. Macrumors warns that this is absolutely not recommended for the casual user, and could have awful results you and I can't even guess at, but it's definitely an impressive trick. Makes you wonder why Apple couldn't have just updated all of the older machines via a firmware upgrade, doesn't it? [Macrumors]