For months the upstream Linux work has been happening by ARM Holdings with GCC compiler support, LLVM/Clang compiler support, Linux kernel support, and Glibc, among other components. This work has now all been merged into the respective mainline code-bases so it's really up to the distributors now to have a hand at making their software 64-bit ARM ready.
Linaro has been working on the initial 64-bit "ARM64" Ubuntu/Debian port and they've been successful in doing so. Announced earlier this morning on the debian-devel list is word that "there is now a bootable (raring) image to download and run."
Canonical wanted Ubuntu 13.04 to be 64-bit ready (the "Raring Ringtail" release) and now thanks to work by the ARM-focused Linaro guys, they're on track.
With this initial AArch64 bootstrapped OS, everything was built against the AArch64-compatible glibc 2.17, there's still a bit of work left to make the rootfs usable for native builds, multi-arch cross-building and the build-profile mechanism works for bringing new ports up from scratch, all packages/sources/tools are in the public repository and should be reproducible. Another highlight is that bringing up 64-bit ARM is fully multi-arch ready so armhf packages for both 32-bit and 64-bit ARM can co-exist on the same system.
Having hit this milestone of a working 64-bit ARM image, Linaro is hoping the distribution developers of Ubuntu and Debian will now take over development and maintenance themselves. More details can be found with this mailing list post.
While 64-bit ARM hardware will be even faster, if you're interested in seeing what the very latest ARM Cortex-A15 performance is like, read Benchmarking Ubuntu Linux On The Google Nexus 10 with the extensive x86/ARM Linux benchmarks included.