I see in Xcode that there is a build setting for architectures and the options are either 32 bit universal or 32/64 bit universal. I understand that a universal binary is a PPC binary combined with an Intel binary that can then run on either system. Is the 32/64 bit universal then 32 bit Intel, 64 bit Intel and 32 bit PPC all combined together? I would assume then on a 64 bit system that it would use the 64 bit version and the 32 bit version when it couldn't. Is there any problem with backwards compatibility? Is the technology behind these universal binaries constrained to Mac OS X? Or is the tool available on Linux and Windows?