For those of you using Macs with an Intel GPU and Lion OS, there’s a bug in the current releases of OpenSCAD that makes polygons and objects disappear when you try to intersect or difference them. The only one that works is union, which isn’t very helpful by itself.
The maintainer of OpenSCAD has released a workaround for the problem. Download the development version 2012.01.14, and install that. Then go into preferences -> advanced, and check the box that says “Force Goldfeather”
This selects the Goldfeather algorithm from the underlying OpenCSG library, and though it has some visual artifacts, it should be enough to keep you going on your projects on the Mac.