I have checked in the X86-64 backend support. It's functionality complete but not tuned for performance. Comments / patches welcome.
However, at this point it is difficult for most of you to make use of it. The current llvm-gcc builds do not support the -m64 (or -arch x86_64) option so there is no easy way to compile for X86-64. If you really want to try it out, you can manually replace the following lines in the llvm bytecode files from
target pointersize = 32
target triple = "i686-apple-darwin8"
target pointersize = 64
target triple = "x86_64-apple-darwin8"
Substitute appropriate strings for Linux, etc.
Pass the modified the bytecode file to llc and it should generate 64-bit assembly. Alternatively, use the -mtriple= option without modifying the bytecode file.