Many source distribution will compile fine for both i386 and PPC Macs. For those that don't, I keep this function in my .bash_profile:
ub() {
echo -n "Cross-compiling "
if [ $1 = "on" ]
then
export LDFLAGS="-Wl,-syslibroot,/Developer/SDKs/MacOSX10.4u.sdk -arch ppc -arch i386"
export CFLAGS="-isysroot /Developer/SDKs/MacOSX10.4u.sdk -arch i386 -arch ppc"
echo enabled
else
unset LDFLAGS CFLAGS
echo disabled
fi
}