Ahh, but what about potential for server-side languages to execute natively in a browser?
Gestalt is an interesting development on that front: taking advantage of Microsoft's Silverlight Dynamic Language Runtime (DLR) to bring Ruby and Python over to the client-side.
A future of <script type="text/ruby"> could be groovy.
window.alert "Hello, World!"
Though I'll be a lot more excited about a solution not dependent on a plugin. Silverlight is far from ubiquitous. Even on Microsoft's own OSs, browser support is not universal. And no love for Linux at all. That's some pretty major shortcomings.
But interesting, oh yes.