I've wondered about this as well. The insulation would probably need to
be something likehttp://www.ybsinsulation.com/superquilt.htm, a
combination of reflective layers kept separate by layers of foam to
prevent conduction of heat. If it were purely foam insulation it would
probably result in a kettle that is unfeasibly bulky.

The kettle could just be double skinned (+vacuum) like a thermos.