Yes, of course. The Stopdesign themes are as well under a CC licence as they are just adapted for Zenphoto but I fear no one really cares about these).
But note that a theme consists of two parts. The code (meanin mostly the PHP function stuff as HTML/CSS is mostly not covered anyway) and the design of your theme.
You can put the design under CC by NC but you can't most likely put the code because that involves code provided by Zenphoto. And any code that is based on code from a GPL licenced script is also GPL. Which is of course so with a theme (short explanation).
Also CC is not meant for software. So you can't put a plugin under a CC licence because of that open source code requirement.
http://wiki.creativecommons.org/FAQ#Can_I_use_a_Creative_Commons_license_for_software.3F
http://creativecommons.org/software
I think we probably should add a special licence part to the theme and plugin description files.