When building the xcode project with xcodebuild,

the linker fails because it has no directions where to find the i386 versions of the system libraries. Despite the QMAKE_MAC_SDK variable being set in the project file (and it works as expected under i386) it is ignored for the i386 build on the PPC platform.

A work around for this problem was to replace the QMAKE_MAC_SDK line by the following three lines:
QMAKE_CFLAGS += -isysroot /Developer/SDKs/MacOSX10.4u.sdk
QMAKE_CXXFLAGS += -isysroot /Developer/SDKs/MacOSX10.4u.sdk
QMAKE_LFLAGS += -Wl,-syslibroot,/Developer/SDKs/MacOSX10.4u.sdk

A real fix would however be qmake using the QMAKE_MAC_SDK variable when compiling i386 versions on a PPC platform.