Alex Smith, a student developer involved with Google's Summer of Code, took up the challenge of porting the 32-bit Haiku to 64-bit. He was successful and got kernel and user-space components working in the x86_64 mode.
This morning he's written his final progress report concerning the GSoC project. "Since the three-quarter term report, I have continued porting userland servers and apps. The app server is fully functional, as are Deskbar and Tracker and a few other apps. I also cross-compiled all of the basic development optional packages (GCC/Binutils, autotools, make, etc.) for x86_64."
Haiku x86_64 is just like a normal Haiku desktop, but that many of the apps and drivers have yet to be ported -- most of the time the 'porting' process just comes down to fixing a few compilation fixes.