Update: Zune High Memory Usage Problem Resolved--Windows Home Server PP1 cause
After a few days of troubleshooting, I narrowed Zune dekstop software memory usage issue down to Power Pack 1 for Windows Home Server. Apparently there is a bug in PP1 that causes all media player software including Zune and iTunes to have all kinds of issues, including memory usage problems.
I forgot I had installed a PRE-RELEASE version of WHS Power Pack 1 prior to the Zune memory issue, and this was when the Zune software started leaking memory really bad. I uninstalled Power Pack 1 PRE-RELEASE, and the Zune software started working properly again on all 3 computers.
After I uninstalled WHS PP1 pre-release from my Windows Home Server (which stores all my media), I downloaded and installed the full OFFICIAL release of WHS PP1, started Zune on a couple of workstations, and the memory problem resurfaced all over again. As soon as I started the Zune desktop software, the zune.exe process memory would quickly use up all available memory within 3 minutes of launching the Zune software.
So it didn't matter if I was using WHS PP1 pre-release, or the commercial release version. Either way, whenever I have PP1 installed on my Windows Home Server computer, the Zune software doesn't work properly. So I've uninstalled it completely and waiting for Microsoft to fix the PP1 issue. Without PP1 on my server, my Zune software works great.
If anyone is experiencing this issue and they are running Windows Home Server with Power Pack 1 installed, uninstall PP1 and I'm sure that'll solve the problem.