I bought my wife a MacBook Air with Mac OS Lion, and when I tried my favorite Mac feature it didn't work...
So I googled it and I found the solution for this big "problem".
To make screen zoom work like it did in Snow Leopard, regardless if you are using Trackpad or Mouse, you have to:
1 - Go to System Prefrences.
2 - Click on Universal Access.
3 - Click on Seeing Tab.
4 - Inside Zoom section, Click on Options.
5 - Check the checkbox at the bottom that says: Use scroll wheel modifier to zoom ^
Thats it. You should now have your screen zoomed by holding Control + two finger slide up/down. I have tested it on my MacBook Air's trackpad and on regular trackpad.