Last year I wrote in a comment that Flex is proprietary. One of the new features of Flex 3 (currently in beta) is that it will be Open Sourced, see this (old) press release. I think this is great news, since the degree of penetration of Flash on the desktop is extremely high, and the GUI’s you can create are stunning nonetheless. Have a look at what Laszlo already did by integrating Flash components into their AJAX framework, now Adobe makes its move!
So, I take back what I said about Flex. Flex is great. I’ll see whether I can whip up a demo on my server (if I can find the time, so bear with me)…