I disabled SELinux and now I can see the outside world and use Yum, so DNS works. Odd.
Anyway, I think I am making this more complex than need be. I have a small peer-to-peer network in the computer lab with around 16 machines. The network is a basic wireless setup. DNS is provided by the ISP. I use DHCP to assign IP addresses on the LAN. I did a text-mode,custom install of CentOS. For the CentOS box, all I really want to do with it is set it up to host shell accounts for the students to access via Putty from the XP boxes. I therefore set the CentOS box with a static IP outside the range of my DHCP. With SELinux off, I can run Yum and fetch updates and extra packages that I may want. This works fine as is.
What I do not get is host name resolution on the LAN side. For example, I can ping a local machine by IP address but not by computer name. Likewise I can ping from an XP box to the CentOS box by IP but not by the computer name. XP to XP boxes do work by computer name. The problem sounds more like ARP at this point and issolated to the CentOS box.
Of course, it's not really a problem as the box works fine as a shell account system, which is all I really need.