Could you expand on that? Are you saying that s390x can run binaries compiled for s390 and that today binaries are being compiled to s390 for the purpose of being run on s390x?

Yes to both (at least for user mode code, or "problem mode" in IBM parlance. Kernel and hypervisor code is 64-bit only on newer chips). There's something like a 30% average memory savings for 32-bit code, so if your program its in 2GB, it's a win on these massive machines that'll be running 1000s of VMs at close to100% load. Nice for your caches too.