(This required hacking Apple's installer to ignore machine checks to avoid the "Mac OS X cannot be installed" obstacle. The method to do that is here: http://forums.macrumors.com/showthread.php?t=371302 )
This got the latest, slightly updated 10.5.5 system (with the energy efficient bulb in the System Preferences) on my first generation Air.
However, I was disappointed it didn't show "Four Finger gestures" in the Trackpad system preference, just the old standard gesture. I had a hunch it was possible though, so...
I then examined the kernel extension "AppleUSBMultitouch.kext" (the one installed with the new 10.5.5 unibody OS X).
This Info.plist of this kext contains several entries for what looks like different makes of trackpads. I noticed that some of them had two keys that the others did not:
I didn't know which model trackpad my Air has and it didn't really matter, as I copied those keys to all entries.
Once the AppleUSBMultitouch.kext was modified (and permissions restored), I rebooted and, hey presto!, there is the Four Fingers entry in the System Preferences.
Four finger Exposé on my January 2008 MacBook Air now works just as it does on the unibody MacBook.
(Secondary click, where you click the bottom right corner of the trackpad to get a right-click, is also there, but only seem to work in combination with the Air's physical button - haven't worked that one out yet, and don't really care as I am used to soft two finger clicking for right-click.)
I suspect this would work on all later model multitouch trackpads.
Source: MacRumors Forum