I'm having this same problem. I'm running 10.5.5, and I was surprised to see many DesignJets listed in the Printer Driver list box, but not the DesignJet 500. So I installed the 700MB+ of the extra HP printer drivers from my Leopard DVD, did an Apple Software Update, and rebooted for good measure. Still nothing.
I guess I'm just going to install the Windows driver in my Parallels environment for now, but are folks from Apple and/or HP working on this?