I'm currently sitting in the Next Generation Client Platform panel, in which four presenters (and a chair) are discussing what the next generation clients are. Interestingly, there's only one presenter on the Rich Client Platform (Jeff McAffer) and the other three are discussing browser-based applications. There's also talk of OpenLaszlo as an infrastructure for developing applications that can allow either a flash-based or DHTML-based application. There's also a Yahoo! library for rich internet applications; so to avoid the problems of reinventing the wheel every time, you can reuse what's been done before.
Jeff pointed out that there's a range of activities that applications need to do. As the workflow becomes more complex, it's necessary to have more and more interactivity within an application. A general consensus seems to be that the interactivity on rich internet applications are getting much better than they were, but perhaps the higher-end applications require a client on the system rather than an internet based one.
One thing's for sure; there's a lot of interest in AJAX and other interactive applications on the web client. Basing your application on a framework (like Laszlo or JSF) seems to be the best way of doing it; you want to leverage interactive implementations, but not by editing HTML by hand. I think the introduction of AJAX has really woken a lot of web-side applications that it's no longer good enough to write the whole stack yourself. Building on top of frameworks (that can be tested, debugged and upgraded) is the only manageable way to go.