Om Malik posts on GigaOm that the "Web monster" has been unleashed and is "gobbling up desktop applications, chewing them up and spitting them out as a Web applications," with the exception of the instant messaging. Browser-based IM clients like Meebo are gaining some popularity, but the popular IM clients have become aren't going away, Om concludes. They evolving into rich communications hubs that couldn't be easily delivered as Web apps.
The browser is the new desktop, but that doesn't mean that desktop clients become dinosaurs. You'll have full and light versions to accommodate the different platforms, at least until bandwidth and synchronization issues are resolved. Given the ubiquity of IM and technologies like Flash, you can expect then to be on any system you might use, including mobile devices.