I was reading through Reddit, and I found this surprising news. I think open sourcing Flex is a master stroke by Adobe to promote its rapid adoption. Now, I think Rails and its other brethren have serious competition. Flex brings an entirely new level of UI sophistication to webapps. Of course, I am not qualified to comment upon how good it is as a framework, but I generally like Adobe products, and I'm guessing they have a good product on their hands again.
That brings the question if they will open source the Apollo platform too. If they do, they have a double-whammy competitor to Rails - Flex takes on Rails itself, and Apollo would take on Joyent Slingshot. Of course, I am aware of the fact that I am talking as if Slingshot is already successful - it's not, but I have to admit latching on to Rails platform is a smart idea, and it would be interesting to see how the this competiton between web frameworks plays out. Fun times ahead :)