The solution I discovered was trivial but quite annoying:
Apparently, this server must have originally been an ESX embedded appliance or something along those lines. The BIOS claimed to be booting from the drive I was installing ESXi to - but that was a heinous lie! The hard drive option in the BIOS boot order had an even deeper option to boot from some flash memory or some such nonsense, which is where the old install must have been located.
Once I finally discovered this, I simply disabled that flash card and everything went swimmingly.
It doesn't sound like thats what is happening for you, ldaprat, but I thought I would share just in case.