Since Google App Engine will soon full support Java:
Would it be possible to run Ruby on Rails on Google App Engine? Or the limitations imposed by the AppEngine runtime will affect the JRuby implementation?
What about other languages such as Groovy, Clojure, Scheme?
Are there any effort to support .net and C# in JVM?? I think this would create a NEXT level on webdevelopment.
GAE has plans to support JRuby on Rails, AFAIK.
.NET and C# already have their own VM, and can run under Linux via Mono... if you're reaching the point where you want a low-level language on the JVM (C#), just use Java, and if you want to use .NET, you're better off staying in the Microsoft world.
It looks like the plan is to support those technologies eventually.
http://googleappengine.blogspot.com/2009/04/seriously-this-time-new-language-on-app.html
If you can use the new technologies with GAE to accomplish what you want to, I think it would be in your favor to do that.
This is already possible. Its not perfect, but I would expect rapid improvement.
More information:
JRuby is already supported, insofar as GAE/Java supports any JRE language. The main difficulty - something that can be overcome by users, rather than requiring Google support - is making ActiveRecord work with the Google datastore APIs.
Unfortunately, I'm not familiar with Ruby, so I can't speak as to how difficult this is. I'm sure there are people already working on it.