Linux x32 is the effort for a native 32-bit ABI for Intel/AMD x86_64 systems but where 32-bit pointers are used to reduce the memory footprint while still allowing for x32 programs to take advantage of the rest of the 64-bit benefits. There's x32 support within the Linux kernel, GCC, glibc, binutils, and even LLVM/Clang.
There's build instructions from the x32-abi web-site for those wishing to manually begin building out x32 ABI support, but to date there hasn't been any major distributions beginning to ship an x32 flavor. Ubuntu does have x32 plans, but for now it's all talk.
Meanwhile, Mike Frysinger of Gentoo has released an initial x32 release candidate of a Gentoo Stage 3 installation for x32. The x32 ABI is the default while still bundling support for the x86 and amd64 ABIs. This RC is using glibc 2.15 with the support back-ported rather than the fully-supported glibc 2.16, but with the ABI compatibility, the upgrade should go smooth. "Once glibc-2.16 is released upstream, we should have x32 stages released alongside the existing x86/amd64 stages."
More information in this gentoo-devel mailing list post.