Oh RHEL 5.5, there are 31 ACPI PCI hotplug slots by default:
acpiphp: Slot  registered ... acpiphp: Slot  registered
Is there a way to increase this number? I haven't been able to find an argument to supply to modprobe, or a sysctl knob to tweak, but I know there must be ways to get more slots based on some Google sleuthing.
(For the curious, this is just preliminary experimentation to see how many virtual disks I can hot-add to a running KVM guest.)