Josh Holmes has posted a set of slides and detailed explanation for a talk he and James Ward did about architecting rich Internet applications. The post and the slides are well worth a read. One of the things that has been both fun and terrifying to watch as Flex evolved was the increasing complexity of the applications. And with both Flash and Silverlight you have a lot of visual options at your disposal so it's easy to focus on flashy visuals without architecting both under the hood and the overall user experience.
That's one of the things I like about Josh's slides, it covers both the design architecture as well as the application architecture. I've started to see this a lot with AIR applications. People spend a ton of time architecting the visual aspects but they aren't thinking about the fact that this application is going to be open for long stretches of time so they haven't thought out things like memory consumption or destroying objects - things that should all be part of a well architected application.
Both the post and the slides (below) are excellent and well worth the read for anyone building RIAs.