not necessarily related to OP, but thought, this might be of interest:
Rhino
I personally am following David Flanagan's incredible book : JavaScript : The definitive guide; I would suggest this book be read by everyone who wants to use JS regularly ... It demonstrates way too many things that makes our lives easier at work. I saw the presentation on YouTube on Good Parts, perhaps some day, I'll get it, for now, I am in love with Flanagan's book, always refer to it, when I need some help.
Personally speaking, because of the dynamic nature of JS and with some good practices, you can write quality stuff talking back to server.
On other technologies, I would have vouched for Flash + Adobe Flex, but the problem I have with Flash is that its owned by Adobe, Had it been open source and the VM would have been spec'd out the way Java VM is, It would have been readily available everywhere (well it is, but still not on mobile as far as I know); Plus the development cycle is much faster with open source IMHO.
Trilochan.