I've got a machine here that was happily running Linux + VMware Server, both 32 and 64bit guests. I previously had ESXi 3.5u4 on it, but the onboard NIC was trouble.
I installed ESXi 4.0 and the SATA required no hackery, and the NIC worked, and I was ecstatic.
Then I tried adding my VMs back to it via Converter. "Host doesn't support 64 bit guests".
I figured that had to be a goof, some problem with Converter.
Set up a new Vista64 guest from scratch, began install, crash "Your computer does not support 64 bit mode". WTF?
HP dc5800 with a Core 2 Quad Q6600. Has VT.
Why would it not be able to run 64bit guests under ESXi 4.0? It ran them under 3.5u4 and under Server 2.0 + Linux host.