I've been to the sane site and looked at the backends. The HP 3210 is supported by hpio and there is a note that hpio is no longer needed because the functionality has been rolled int hplip. yum shows hplip is installed: hplip.x86_64 1.6.7-4.1.el5.4 installed
iptables allows all outbound and established.
Anyone successfully scanning from an HP Photosmart network printer?
RHEL/CentOS have very old packages for HP printer/scanner support, although there will be a vastly updated version available in 5.6. I just got a new All-in-One e709a to work across the network with the packages from rpmforge.
You can use rpmforge through yum, or download the packages directly at http://packages.sw.be/hplip/
Once installed, run (as root) hp-setup
Barry