It took a bit of experimentation but ESXi 4.1.0 is now running on an ancient (4 year old) HP.
A direct install of ESXi 4.1.0 on my old HP kept erroring out (couldn't access the SATA storage). An attempt to build a custom install disk, reusing the oem.tgz file, didn't work. Building yet another oem.tgz file was obviously going to be the method-of-last-resort because it's been a year since the last time and that involved two days of trial and error.
So, finally, I broke down and started reading the forums. Surprise! VMware has an upgrade bundle for moving from 4.0 to 4.1. I downloaded the zip file, put the test system in maintenance mode, and ran the esxupdate tool on the command line. It installed without a hitch.
Same for the house server. It's now running 4.1.0. I was anxious that it'd replace all of the drivers (the house server was built using last year's custom install disk) but it appears that it didn't touch any of those. The hypervisor can still see both datastores and can still access the network.
Note to self: If you have to install this again, remember that the system needs to be put in maintenance mode BEFORE attempting to run esxupdate.
Note to Dave: I think that the 4.0 custom disk is the same one that we used to build your system. This should be a quick upgrade to your system, too.