VMware ESX / ESXi 4 does work out of the box on the HP Proliant ML110 G5. I know, I'm using ML110's with ESX 4. However, I still see posts like this http://communities.vmware.com/thread/163029 were people have problems with this ML110 and ESX 4 and even ESX 3 combination.
Now, if you look at the link to VMware Communities, you will notice in one of the screen shots attached that the CPU is a 1.8GHz Dual Core. The ML110 G5 comes in more than one CPU, so beware that when you decide to get one of these HP Proliant ML110 G5 for running ESX 4, make sure you get one with the Intel Xeon 3065 CPU that runs at 2.3GHz. The reason is because not all Intel CPU's shipped with the ML110 supports Intel VT, and this is a requirement for ESX 4.
Now I know you're probably saying: "The guy in the post is not using ESX 4." My point is simple: It doesn't matter what version of ESX the post refers to. If you want ESX 4 to run on the ML110, make sure you have the correct processor.