OK, many people are starting to wonder about Mac OS X universal binaries. The good news is that you can start playing with universal binaries already with the release candidate for Qt 4.1.0. Here's the steps, assuming you are on a PPC machine.
configure Qt with these extra flags:
-universal -sdk /path/to/MacOSX10.4u.sdk
After which you can simply build Qt normally. BUT, If you are one of those people who have upgraded to Xcode 2.2, you will have to edit QTDIR/mkspecs/features/mac/sdk.prl to make it look a little more like this: