Why Did Full Screen Zooming With Scroll Wheel Stop Working In OS X Lion?
Inconsistencies with full screen zoom appears to be a bug in the first release of OS X Lion. It works for some users, but for most users zoom will only work occasionally. Turning zoom off and then back on sometimes helps to get it working again.
To toggle the zoom settings in OS X Lion, go to System Preferences -> Universal Access. Click on the Seeing tab and then select Options. At the bottom of the window there is a checkbox that says "Use scroll wheel with modifier keys to zoom". Uncheck and then check this box. Zoom should start working again temporarily.