Rather than committing to a given scripting language, I suggest we instead look at the possibility of a platform independent virtual machine. Authors create scripts in the language of their own choice and compile to the VM. In otherwords we send machine code over the net NOT scripts. This also has the benefit of protecting the intellectual investment in the program as expressed in the higher level language.
An efficient stack oriented machine with support for memory management, and garbage collection, together with support for inheritance, would make a good Msc project - is anyone interested?
Alternatively, we can go for a risc vm, and work is underway on this at MIT. This offers the possibilities of higher performance, and would be useful for say extending the browser to support new kinds of graphics formats for inline images.
The API between browser and VM needs to be pinned down. Some of the ideas aimed at augmenting forms are:
Note I don't think the VM should be allowed to send messages, at least for untrusted "scripts". A mechanism for trusting scripts based on digital signatures is going to be a useful extension.