Chris Keene wrote on his Blog a personal definition of RIA's:
"Rich Internet Applications match the responsiveness of traditional desktop apps by minimizing web page refreshes. RIA taps into the collective power of the Internet to supply widgets and services for building web clients, like rss feeds, Google maps and Youtube. The goal of RIA is not merely to emulate a PC GUI in a browser (aka the Silverlight sell-out), but to deliver browser-based clients which far outperform PC GUIs in speed and functionality."
Lot's of people indicate that now is the time to use RIA's in the enterprise. There are more releases of RIA products (Dojo just had its 1.0 release, before it were 0.x releases). There are currently 58 mature RIA products to choose from. Which one to choose is another question, but the time is here to think about that!
