Creating a new project with Xcode 3.2.2 sets a number of build options by default that are worth setting on older products. One that I discovered by accident is the Validate Built Product option.
I cannot find a list of the checks that this option performs but it did discover an error that I suspect I would never have found on my own:
iPhone/iPod Touch: icon.png: icon dimensions (56 x 56) don't meet the size requirements. The icon file must be 57x57 pixels, in .png format (-19014)
Sure enough my icon file was 56x56 pixels which I guess is enough to get an app rejected by Apple. I don't know what else it may be checking but this is definitely an option worth turning on for distribution builds.