I've not been following the .NET smart client saga very closely but if the Microsoft TaskVision sample (via Chris Sells) is state-of-the-art it suggests that the original vision has been watered down somewhat. I say this because I went to run the client but stopped when I noticed this requires downloading and installing an MSI package. I really liked the concept of being able to run clients, from a URL, in a "sandbox" which uses Isolated Storage on the client machine to persist data without having general access to the file system.
Running an msi package supplied by the producer of a client breaks security. Ideally, when clicking on a link to run a true smart client, I'd like to be presented with a dialog telling me which permissions the smart client is asking for and giving me the opportunity to choose whether I accept them, or even accept a subset if the client offers this alternative.
Another feature of the hypothetical zero-install client is that it doesn't involve that horrible feeling I always have when I install software on my system, that it will either conflict with something already installed or I'll never be able to uninstall it without breaking something, or worse still that I end up in MSI hell in which every time I run the client it fires up the MSI installer to fix some part of the installation, only to fail because the version of the software installation package on the corporate network has changed.