Unfortunately, this is incorrect. Apples own information on Snow Leopard's enhancements reads, "All Mac notebooks with Multi-Touch trackpads now support three- and four-finger gestures." (emphasis added)
This raises the question, what's the difference between a multi-touch trackpad and a regular one, and which models have it?
The multi-touch trackpad was introduced with the first MacBook Air in early 2008. Not only does it allow two-finger scrolling like older models, it also allows advanced three-finger gestures like swiping to go back in Safari.
One month later, the early 2008 MacBook Pro received the same trackpad, with the same gestures. The multi-touch trackpad gains this new functionality because it has an embedded controller chip, identical to the one in the iPhone and iPod Touch, which allows advanced input from more than two fingers at once.
Later, the unibody MacBooks and MacBook Pros debuted with multi-touch trackpads, but also introduced new four-finger gestures, which will not be officially supported in the older MacBook Airs and MacBook Pros until Snow Leopard's release.
The original MacBook Air and early 2008 MacBook Pro are the only machines which will gain additional gestures via Snow Leopard. The only reason these notebook models are able to gain these gestures via software updates, while earlier MacBook Pros and all plastic MacBooks are not, is because they possess the multi-touch controller chip in their trackpads.
Just to break it down, this is a list of the only, and I mean only, notebooks that support multi-touch gestures, either now or after Snow Leopard:
MacBook Air (all models)
Early 2008 MacBook Pro
Late 2008 17" MacBook Pro
Unibody MacBook (all models)
Unibody MacBook Pro (all models)
If you have a MacBook Pro manufactured before early 2008 or any plastic MacBook, then Snow Leopard or not, multi-touch isn't coming your way...
...But if you're like me and you do have one of the earlier multi-touch enabled machines, you don't have to wait for Snow Leopard to use the newer multi-touch gestures. Since 10.5.6 was released, it's been possible to do some creative hacking to enable four-finger gestures on original MacBook Airs and early 2008 MacBook Pros.
The procedure, found in the MacRumors forums and courtesy of MacRumors forum member fjk, is surprisingly simple. Standard disclaimer: if the contents of your System folder frighten and confuse you, or if when you hear the word "kext" you reach for your Browning, you should proceed at your own risk.
First, download a modified AppleUSBMultitouch.kext file. Navigate to System/Library/Extensions, and remove the old AppleUSBMultitouch.kext (you will need to type in your admin password).
Move the modified AppleUSBMultitouch.kext into System/Library/Extensions. You'll most likely have to type in your password again.
This next step is critical: repair disk permissions using Disk Utility. If you don't, after you restart your trackpad will not function.
Once permissions are repaired, restart. Success!
I can vouch for this technique, as it's worked perfectly for months on my early 2008 MacBook Pro. Note that you will have to repeat this procedure for every OS X dot-update -- if 10.5.8 comes out before Snow Leopard, you'll have to do the above hack again to re-enable four-finger gestures on your machine.
Apple MacBook Air 13-inch (early 2015)
Apple MacBook Pro 15-inch with Retina Display (mid 2014)
Apple Mac OS X Snow Leopard