We've just acquired a pair of HP DL380 G6s for our vSphere 4 test lab, they have the new Intel Nehalem Xeon CPUs and chipset with all the latest virtualisation features. Plenty more to come on all that but first of all here's something you won't find in the documentation. Having just installed the ESXi 4.0 hypervisor on the servers the classic question came up from one of the junior technicians - "what's the difference between ESX and ESXi?". Someone else then answered that ESXi doesn't have local console access like ESX does, so all management has to be done from the vSphere client. Although this answer would probably score you points in the VMware exam its not exactly true, as I was able to prove.... This is the same procedure which worked in ESXi 3.5, it still works in 4.0 too. Although VMware do provide a remote CLI appliance that gives you much of the scripting support you'd get with ESX sometimes its much easier to use the console, so once you've got your ESXi4 server up and running at the yellow and black screen do this:
1) Press Alt-F1 2) Type "unsupported" - note you wont see this echoed on the screen, but after you press return you will be prompted for the root password. 3) Enter the password 4) Marvel at "tech support mode"

Whilst you're in there why not give yourself remote SSH access too:
1) type "vi /etc/inetd.conf" 2) remove the # before the SSH line, save and close 3) enter "services.sh restart"
The VI text editor is a good example of how ESXi has been stripped down, its the old school *nix text editor and you will probably have to Google the instructions for it if you haven't used it for a while, but forget about something fancier like Nano. Like its says on the warning page though, this is "unsupported mode", so if you break it don't expect to get any support, you'll just have to re-install. ESX Server was has always been in effect a glorified Linux server, with all the admin and configuration options you would expect from a Linux server. The problem VMware had was that it was getting rather bloated, a standard ESX 3.0 install would use near enough 2GB of disk space and took a good few minutes to boot. In response to this VMware developed ESXi, a lean, mean hypervisor with minimal disk and memory footprint which could then be "embedded" on a server. This fitted in with VMware's vision of the future of virtualisation, where server boxes just become plug and play vSphere resources. It also happened to reduce the functionality enough to not annoy all those people who had paid for ESX when they started to give ESXi away for free :) Reading the VMware documentation you get the impression they rebuilt the ESXi hypervisor from scratch as a dedicated virtualisation OS. In fact it looks more like they went through their last Linux build and simply removed all the stuff which wasn't essential. As a result there is still a Linux based OS under there, and a console you can access, but a lot of the packages you might expect as standard won't be available.