OK, I found out what it was, but I'm not thrilled with the solution. I had to uncheck "Hide Protected Operating System Files" and the directory became visible in Explorer again. I say reappeared because I could see it and other files in the Users directory (such as Default User) before and hadn't altered that settting, so I don't know what changed. It's almost like it wasn't set as a system file and I could see the directory's contents, but then something changed that caused it to become a system file and became hidden due to the folder options. Anyhow, note the Hide Protected Operating System Files option is in addition to the option to view hidden folders, so there were 2 ways to influence what can be seen in Explorer.
I used to be a sys admin, so I know this is going to sound like secret code for "I'm about to tell a whopper", but I honestly didn't change anything and didn't execute any system commands (at least knowingly) that should have changed this directory's state but something happened that I can't explain. With that as the background, I'd like to reframe the question in a few parts:
*) Is c:\users supposed to be a system file?
*) If not, how do I changed it back?
*) If so, can I/should I change it to not be a system file so I can hide system files in Explorer again without problems?
I am happy with Windows 7 and know there's new things to learn, but this one was a bit of a surprise to me. Thanks for your help