Merged into the Linux 3.4 kernel was x32 ABI support, which for x86_64 processors basically means taking advantage of all the modern 64-bit features and increased register set but while still mostly using 32-bit memory pointers for a smaller memory footprint. Glibc 2.16 also picked up x32 support as did GCC plus LLVM/Clang now has the support. GDB also picked up support this year for debugging x32.
The first tier-one Linux distribution offering up any x32 support was a Gentoo x32 Stage 3 installation. While in an unofficial manner, joining the party now is a Debian-built x32-supported Linux kernel and some other x32-built packages.
Since two weeks ago there has been a x32 port Debian Wiki page. Linked to from there is an archive with a Linux 3.6 kernel built with the CONFIG_X86_X32 option enabled for the necessary kernel-side support. There's also various x32 Debian packages from there too. The command is also given for setting up an x32 chroot of Debian if so desired to experiment with the new Linux ABI.
While x32 is attractive due to the smaller memory footprint, it's not yet picked up much steam with most Linux users and developers being content with just using x86_64 packages. Linux x32 benchmarks are forthcoming.