I was reading my way through the Internet and came across some interesting stuff about Smart Client Architecture. It seems that the definition of a Smart Client is not so clear. On the Microsoft site it says:

Smart client (n) Definition: Smart clients are easily deployed and managed client applications that provide an adaptive, responsive and rich interactive experience by leveraging local resources and intelligently connecting to distributed data sources.

But then it shows the following picture:

As you can see there are two features that are unique to a Smart Client:
a) Online/Offline support
b) Device Adaptability
Without these features a Smart Client is really nothing more than either a Thick or Thin Client.

Personally I feel that from an architectural point of view offline/online support is much more important than device adaptability. Therefore I think that the definition for a Smart Client should read:

Smart client (n) Definition: Smart clients are easily deployed and managed client applications that provide an adaptive, responsive and rich interactive experience in both an online and offline scenario, by leveraging local resources and intelligently connecting to distributed data sources.

More interesting reading on: The ServerSide.NET and Clemens Vaster's Blog.