Combining the power of the cloud and the client allows developers to have the best of both worlds' easy deployment and full client capabilities. Salesforce.com's cloud platform, called Force.com, and the Flash Platform are two proven and reliable choices for building these types of Rich Cloud Applications. Last week I co-hosted a webinar called "Developing Rich User Interfaces on Force.com Using Adobe Flex" in which Ryan Marples (from Salesforce.com) and I walked through the two platforms and how to use them together to build great software. If you didn't have the chance to join the webinar then please go watch the recording and let us know what you think.
James Ward, preeminent Flex Evangelist has an excellent write up on the webinar app that was demo'd. Includes useful source for manipulating Flex items.