Thanks Tyler for guiding me to the post by David Malouf where he writes about the foundations of interaction design. I like his text and especially the way he frames the question of foundations. I agree that we do need to develop a notion of foundations.
I have done some work myself (of course :-) around this. The latest attempt is in a paper that is today presented at the DPPI conference in Helsinki by my colleague Youn Lim. The paper is called "Interaction Gestalt and the Design of Aesthetic Interactions". In that paper we try to define "interaction" as something separate from both experience on the user side and from properties on the artifact side.
Anyhow, more discussion about what interaction is all about and how it differs from other disciplines is needed, and David Malouf is right that is truly a special area of design, with its own foundations.