Much to my chagrin, I have found that VMware ESXi cannot run 64-bit Guests on all 64-bit CPUs. It turns out that the CPU needs to support 64-bit virtualization not just be able to run a 64-bit host. So, my Dell SC1425 server that handles 64-bit Intel processors will not support 64-bit guests due to the processors not being compatible with 64-bit guests.
To avoid this problem, look for Intel CPUs that support VT (Intel Virtualization Technology) and AMD CPUs that support AMD-V (AMD Virtualization). Another thought is that all modern or new server processors probably support 64-bit guests, but if you are looking to utilize an older server then watch out.