At last I found a solution that solved my issue. The faulty step while using Pacifist was that it installed the "Aspen" packages in the root folder of my primary disk.

As soon as you have the Xcode and the iPhone SDK installed in the root folder, you can't build successfully any project, because of an error of the ibtool.

In order to having the full iPhone SDK running on my MacMini PPC-based these are the steps you should follow. Several information is available online about that, but those infos are quite fragmented:

• be sure to have your Leopard updated up to 10.5.4 or 10.5.5 (it's a requirement of the original iPhone SDK 2.1)
• install the full XCode 3.1 (it's only a 1 GB package)
• use Pacifist to install the iPhone packages although you could install everything in the root folder and then move the folders iPhoneOS.platform and the iPhoneSimulator.platform from the /Platforms folder to the /Developer/Platforms folder. You may need to authenticate as an administrator to complete this action. Please leave the MacOSX.platform in that folder, because XCode 3.1 installation should have already installed such folder in the /Developer/Platforms
• trash the unneeded packages and files put in the root folder. Everything you need has already been installed in the /Developer folder by the XCode 3.1 installer (you may trash About iPhone SDK.pdf, About Xcode Tools.pdf, some duplicated applications in /Applications instead of the right /Developer/Applications, Documentation, Examples, Headers, Makefiles, Platforms, Tools)
• patch the file iPhone Simulator Architecures.xcspec located Developer > Platforms > iPhoneSimulator.platform > Developer > Library > Xcode > Specifications > in order to have a right target architecture available

An alternative way could be to install Xcode 3.1 with its full installer and install the iPhone packages using Pacifist telling it to install them in the /Developer folder instead of the root one, but I didn't try such way, so make it at your own risk.