It appears that for whatever reason, the kernel on my Mac is 32bit/i386, but the architecture is 64bit:
Darwin system.local 11.3.0 Darwin Kernel Version 11.3.0: Thu Jan 12 18:48:32 PST 2012; root:xnu-1699.24.23~1/RELEASE_I386 i386
Though the kernel itself is a universal binary:
mach_kernel: Mach-O universal binary with 2 architectures
mach_kernel (for architecture x86_64): Mach-O 64-bit executable x86_64
mach_kernel (for architecture i386): Mach-O executable i386
Am I mis-reading this or is there something else I need to do to fix it.