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You could use Rhino w/ AppEngine JVM if you like Javascript so much. Rhino actually supports newer version of Javascript (1.7 vs 1.5 for V8). Check out for example: http://www.appenginejs.org/

The AppEngine datastore is somewhat similar to Cassandra (ok, so really HBase, but you get the idea)

You can't run Node.js on AppEngine, but there is http://www.appenginejs.org/ (Javascript on AppEngine).

I'm not familiar enough with Node.js to say for sure, but it's possible you could write a compatible javascript API for AppEngine under Java.

I'd love to see how people are running map/reduce jobs on AppEngine.

There's a mapper API http://googleappengine.blogspot.com/2010/07/introducing-mapp.... It's a layer on top of task queues. No reduce yet.