I haven't tried compiling 64-bit yet. 64-bit wouldn't make it any faster, since it uses 64-bit numbers in only a few places and SHA-256 is a 32-bit algorithm, but it may be convenient for those running a 64-bit OS. If I get a chance I'll try -m64 and see what the problem is.
You can run the 32-bit version on 64-bit Linux by installing ia32-libs. (sudo apt-get install ia32-libs) If we made a Debian package, it could automatically pull that in as a dependency.