With the release of Ubuntu 8.04 "Hardy Heron", there are experimental emc2 packages available for 64-bit systems. However, these packages are not yet recommended for production systems due to reliability problems.
Intel and AMD CPUs which implement AMD's 64-bit instruction set. SMP and multi-core processors are supported (up to 8 total cores).
Install 64-bit Ubuntu 8.04, then run emc2-install.sh from 
. A live CD is not planned at least until the reliablity problems are resolved.
RTAI does not compile the SMI disabling module when building for 64-bit systems. This means that many Intel-based PCs will have the latency issues that SMI causes with no easy way to resolve them.
On jepler's main testing box (AMD X64 3800+), the kernel tends to crash after running anywhere from 1 day to 2 weeks. This system has been reliable under earlier Ubuntu versions with non-realtime kernels.
There are likely to be additional software bugs revealed by the different sizes of integral types on 64-bit systems.