Cabochon options for flameworkers have changed considerably since my original post back in February of 2012. Here is an update to help navigate the world of cabochon systems and tool suppliers.
Changeable Systems for the Artist who enjoys Options:
Flat-back Cabochon Mandrels:
Cabochon Mandrels with a Nub (yes, nub or nubbin are incredibly technical terms):
Zooziis (available in a variety of shapes)
Snap Cabochon System by Patty Lakinsmith: