What do we mean by supple interfaces?Supple interfaces can be characterized by their focus on:
Rich human communication and interpretation strategies (e.g. emotion, social ritual, nonverbal communication, kinaesthetic engagement).
Taking into consideration subtle communication dynamics that require new thinking about system adaptivity and feedback. For example, increased legibility of system moves to help users actively co-construct practice and meaning and push system boundaries in interesting ways.
Privileging the quality of moment-to-moment experience both in terms of design and in terms of evaluation of success of design (e.g. a focus on engagement, pleasure, rapport).
Examples:Example systems can be found in games, affective interactive systems, affective computing, sports applications, relationship-building applications, mobile applications and a range of other non-productivity interfaces.
Commercially, the games industry has led the way in bringing supple interfaces to marketNintendo in particular, with the release of the DS handheld with its touch screen and audio input, and the impending release of the Wii platform with its built-in accelerometer, has provided game developers with myriad opportunities for creating supple interactions with users (e.g. the popular Nintendogs game for the DS, or the body-engaging racing interaction in the tobe- released Excite Truck).
Supple interfaces typically engage the person in emotional/affective ways, with the use of physicality creating a sort of 'affective loop' [Sundström et al.]. An example system that has been designed from this perspective is eMoto - emoto.sics.se. In eMoto, users compose text messages on their mobile phone and then use an extended stylus that can pick up pressure and shaking gestures to express the emotional value of the message. The gestures are translated into colors, shapes and animations in the background of the text message.
Supple interfaces also seek to support subtle relational dynamics between people. For example, in Feather, Scent and Shaker by Strong and Gaver  a set of designs that we would consider supple interfaces are described. For example, with Feather a couple has one device each; the person who is away from home has an interactive picture frame and the person at home a cone containing a feather. When one user is handling the picture frame, a fan starts blowing the feather at home, illustrating affection in as a poetic experience of connection.
References:Strong, R., and Gaver, B. Feather, Scent and Shaker: Supporting Simple Intimacy, In: Videos, Demonstrations, and Short Papers of CSCW 1996 ACM Press (1996) 29-30.
Sundström, P., Ståhl, A. and Höök, K. (forthcoming) In Situ Informants Exploring an Emotional Mobile Messaging System in Their Everyday Practice, accepted to the special issue of IJHCS on Evaluating Affective Interaction.
The workshop description as it will appear in the CHI conference proceedings: PDF