Scott Garvey’s Discussion of Smart ClientsChannel9 has a nice video of Scott Garvey, a Microsoft Smart Client Evangelist, talking about what’s a smart client and what’s not. Scott also discussed some examples of smart clients, e.g. eBay for "power sellers."
For me, things boil down this way: if it’s something I’ve got to spend significant time in, I really want it to be a smart client, otherwise I’m going to get frustrated. For example, Outlook Web Access is great for occasional use from any PC under the sun, but if I’m using it for more than 5 minutes, I’m guaranteed to press a hotkey like Ctrl+A and get nothing like I expect, forcing me to find my laptop so I can get some real work done in Outlook.