We have a HP5500 DesignJet 60inch, it has an IP address, we can access it from our PC's - but it does not have a Mac printer driver so we can't print from the Mac direct to the HP. We also drive the printer using a third party RIP, which has pro/cons vs. using the supplied HP driver software.
EDIT:
Go to the Setup menu on the front panel of the printer.
Select I/O Setup and press enter
Select Card ID and press enter
Write down the hardware Ethernet name and IP address numbers (and perhaps Appletalk name if appropriate)
You should be able to browse the printer from a web browser using the IP address.
Hope this helps!
Stephen Marsh