Dave Wolf of Cynergy Systems has a great read on AJAX and RIA. Dave contends that most decision makers do not fully realize that AJAX is DHTML reinvented. It is an interesting read.
Living in Denial with AJAX by Dave Wolf
FYI, Dave Wolf was on the Microsoft BizTalk Server 1.0 team and on the dev team at PowerBuilder/Sybase before that. He knows enterprise software inside out.
Cheers,
Ted :)
DIGG IT! 
Yes, it's DHTML + XMLHTTPRequest. Check this out:
http://java.sys-con.com/read/249445.htm
Ted:
I've done a good amount of work in both the Flex and Ajax world, and as such, I'm constantly asked by clients and my team when to use what...while I too am a bit Flash Platform biased, I don't always get to make the technology decision with my clients, so like I said, I've had experience with both...low and behold, the reason I left the JSP / HTML world and fell in love with the Flash Platform still holds true (and suddenly came back to haunt me as every unknowing architect started demanding the use of Ajax)...browser / platform inconsistencies.
To that, I created a couple posts to collect my thoughts on the 2 RIA players: Flex v Ajax. You might find a familar position to your own...Flex or AJAX :: A Quick Litmus Test