You've surely heard about Ajax : A New Approach to Web Applications, and what people say about it.
You can read : Dara Obasanjo's blog, or Could Ajax Wash Away 'Smart Clients' ?
In this last article, web clients are opposed to smart clients, considering that MS has failed... It is surely mixing things that have not much in common...
There are several points to consider : first of all, it has always been a nice dream to move from desktop application to web application, but you can notice that even if mail client have really nice interfaces, people prefer using desktop mail clients since you can use it offline, it is faster, have more adapted user interface, and especially user notification features.
Notice that even GMail uses the GMailNotifier desktop application to alert the user when new mail arrive on his mail account. We can be sure that Google won't last in providing extensions for mail clients to handle GMail accounts...
The second point is that you can't do everything in a web client due to security concepts : you have a limiter support for reading and writing file, very limited support for remote connections... There a whole categories of software that you could not implement in a browser without taking serious security risks, especially P2P softwares.
It is a nice thing that Web application becomes a more smart, it is a nice thing that smart application becomes a more web oriented, but this are two complementary domains. Developers must use the best of both depending on their needs...
by Skup