David Hill has a pretty good definition of what a smart client is [link from Chris Sells]... it's especially fitting because of my upcoming MDC talk:

"In addition, the client platform has moved on in the past few years and now includes many different types of client devices, not just desktop PCs. Such devices include PDA's, SmartPhones, Tablet PCs, Laptops, set-top boxes, automotive devices, retail terminals, and so on. Smart client applications can be built to take maximum advantage of the features provided by the host device and tuned to provide the best user experience for the typical users of these devices."

He also has a great check list:

"

• Utilizes Local Resources
• Connected
• Offline Capable
• Intelligent Install and Update
• Client Device Flexibility  

"

This is one of the more clear definitions of smart clients... too often people get too trapped into specific features. Using Office doesn't make your application a smart client, writing with the .NET Framework doesn't make it smart either. The key is to offer a great user experience with compelling functionality.