NOTE: this hint was developed under XCode2.4 and Mac OS X 10.4.8, it probably won't work on pre-Tiger systems.
We just got some MacBook Pros, and our lab is now mixed PPC and i386. We rely on a number of command line programs developed over the years and ported from various BSD-flavored systems. We decided to convert all these programs and libraries to universal binaries so that all the systems can copy them from a single installed prototype. However, the process of converting bunches of makefiles to universal binaries seemed to be kind of a pain, so I wrote a simple program to convert cc(1) to produce them by default. The source can be downloaded here.
This tiny program works by inserting three critical arguments into a standard cc command and then calling gcc with the revised argument vector. It is installed as a separate command called ccub, and optionally it can replace cc so that universal binaries are the default. There's more explanation in the tarball.
[kirkmc adds: Not tested.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061025213851279