Yakov Fain of Farata Systems does a great post comparing the various options for doing Enterprise RIA. Adobe Flex and WPF are on the list. I think we are starting to see a continuum form out of the available choices. On one end is the Rich Client Application space (Desktop) with WPF and on the other end we have plain HTML+CSS. In between we have Flash/Flex, WPF/E and AJAX.
The extremes are not always a good choice. Is it the middle ground? (Pun intended)