In 100% of the cases where I've seen that header on different HP workstation motherboards it has been active when I have needed to use it. We simply plug on the optional passive parallel port adapter I posted about. We use these mainly for attaching an electronic "dongle" that must be present on a workstation for particular expensive medical software to work. Some medical printers are still going strong, and have only parallel port input. So, I am experienced with these adapters in different settings on different HP workstations/Business PCs. Look on page 12 of the dc5800 QuickSpecs here: http://www.nts.nl/site/html/modules/pdf/PC/dc_5800.pdf and you'll see the adapter is offered as an accessory for both form factors of that computer (the Micro Tower and the Small Form Factor).
Those passive parallel port adapters were an optional purchase, and they usually came as a kit with the full height or low profile backplane metal plate you could screw on as needed. The kit has HP Option P/N of KD061AA, and the HP Spares P/N is 465339-001. A screen shot from a tech document is below, and the quote there for the HP technicians is to not recommend the PCI card approach but rather to "Always recommend HP Parallel Port Adapter or HP 2nd Serial Port Adapter, or both, for HP Compaq dc5800/dc5850 models". Take a look HERE.
The "HP 2nd Serial Port Adapter" mentioned above also attaches to a header on many of the HP motherboards as an option. For many of the more recent HP workstations this is the way to add on a serial port (which we still need for some Cisco device software/firmware updating).
I have never seen any HP reference to the parallel port header being present on a motherboard but inactive. That would not make sense from a supply chain perspective. It is the adapter that is present or absent as the option mentioned in the QuickSpecs. For this reason I'm hoping the original poster will spend the six dollars and get back to us that it worked well on his dc5800.
Note that the half height eBay adapter I linked to is for a dc5800 SFF, not the Micro Tower version (which would use the full height metal plate).
