Does anyone know of any manufacturer that lists CentOS as an officially suported OS? Fujitsu and Dell don't support it. I'm specifically looking for CentOS support; vendors that only support RHEL are not sufficient for my needs.
I don't know of any directly, you might have to contact CentOS to find out. But what are you looking for by supported? That their hardware works properly, or that they'll troubleshoot your hardware with a support contract if you have it installed?
If you just want compatibility, anything that "officially" supports Red Hat should run CentOS, since CentOS is Red Hat's code.
In my experience most vendors have trouble supporting anything that isn't Windows, whether they say they support it or not (just adds layers of headache in the process and we end up doing the legwork ourselves). Ninety percent of the time when there's a problem (we use Dell) it's a call to them due to hardware, and for that they want you to run onboard diagnostics regardless of operating system. It's usually not hard to convince them of the necessary new parts to be shipped once you run their diagnostic CD or controller diagnostics, and we don't normally have to call the manufacturer because of misconfigured software, since fully supported doesn't apply to software beyond the OS anyway.
I know you don't want it, but Red Hat is a pretty good guideline.
As far as support incidents go, Dell isn't too bad with their PowerEdge servers when it comes to Linux, they recognize that this is what people want to run. Red Hat is pretty well supported. For diagnostics/troubleshooting, it is a good idea to install the OMSA (Open Manage Server Assist) piece into your system so that you can look at the hardware states.
In my experience with Dell, 90% of the time when I have a bad disk, I just make the warranty call and say "My ESC is foo, I have a bad disk" and they just send me a new one, no questions asked. 90% of the rest of the time I can say "Well I have two systems that are the same and I've swapped drives, and the second system agrees that the disk is bad" and they send me the new disk. Once I had some guy drill me around with re-seating and looking in the OMSA (he was surprised I already had it installed) and firmwares and controller cabling, but eventually he too just sent me the disk.
If your OS isn't supported, Dell will simply ask you to download and boot a diagnostic liveCD (which in my days used to be based on CentOS4). That should be enough to put the OS being at fault out of the picture.