Dan Saffer just published his model for the relationships between Interaction Design (IxD) and the fields around it:
I love how it matches the ideas behind my T-model for User Experience: one can specialize, or go deep (in Dan's case into IxD), there is overlap between fields, and User Experience Design is an umbrella field that embraces a lot of other fields.
For those who wondered: In a comment on the Interaction Design mailinglist, Dan points to the following article about the lack of overlap between Interaction Design and HCI: Just How Far Beyond HCI is Interaction Design? (by Jonas Löwgren). An interesting read indeed!