Excellent, that's exactly what I was looking for. Thanks.
The reason I need this is to allow for styling differently based on the mode. If you have your own container skins, you need to have a module header (for the drop-down menu, module title) and a module footer (for the 'Edit Text' and 'Settings' buttons). These are required for the administration, but not for the public viewer of the site.
However, the containers for these (table rows in the default container skins) are left in place when viewing the site as a public user. Although they're empty, they still leave blank lines where I don't want them. There is a 1px blank line for the header, and a full text-line-height blank line below the module - which I can't have in the skin I'm implementing.
Having a class set in the skin that identifies the mode will allow me to correct this in the style sheet really easily. So, thanks a lot for this!