I read an interesting article on UX Matters earlier today; a very good comparison between the 'old school' page-based Web application metaphor and the emerging rich internet application interaction metaphor being popularised by the later versions of Flash and the XML/DOM combo labelled by some 'Ajax'.
The article can be found here and I would recommend your reading it. David Heller presents a very clear, well constructed overview of the two metaphors; provides the reader with a framework for selecting when to use each appropriately; and discusses some of the advantages to be obtained from the use of RIAs for the particular task of user-intensive Web applications.
It is without a doubt one of the better articles on the topic that I've seen in recent times.
I might as well also mention that the initial swag of articles on UX Matters has been most promising, and I've enjoyed the more academic, journal-istic style of the articles.
[UPDATE: Apologies for the dud link to David's article. For some reason we had an extra http embedded in there, causing it to redirect to Microsoft of all places.]