Before Xcode 3.2.6, the default build configuration for an application was a universal binary that ran on both PPC and Intel Macs. Xcode 3.2.6, however, has a default of Intel only, and there is no universal binary option in the drop-down in the Build tab of the Project Settings.
When I did a new release of MailSteward recently, using Xcode 3.2.6 for the first time, the result, unbeknownst to me, was a 32 bit Intel only build, not a universal binary build. I didn’t expect Xcode to secretly change my project settings, so I didn’t notice what had happened.
Here is how to fix it. Select Other… from the Architectures drop-down in the Build tab of your Project Settings. You can then add your own architecture, as in this screen shot:
Your Architectures Setting will now look like this:
Then clean all targets and do a new build and the result will be a universal binary application.