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The comments are worth reading, IMHO.

Well, Spring home web site is written in Python. It's a good language.

The memory isn't an issue for Google -- anything they'd let you deploy would be limited. It might be a problem to write a useful app under whatever limit they impose, but that's your problem, not theirs. You can probably pay them for more at some point.

The jars is kind of problematic, but with Python you just upload whatever you're importing module wise. I don't see why you couldn't so the same with jars. I imagine they'd make a standard set available to everyone.

The threading is the real problem -- Google doesn't let you use threads or any other system specific thing in the current app engine. I would wager that the JVM is way more complex than the Python runtime, so making a custom Java runtime that disabled all access to system resources, threads, etc.. from Java code is probably a much more massive undertaking than doing so for Python.

Mmmm... Is really that hard to achieve the limit on thread usage with the current JVM?. I though you could just not grant permissions to create or modify threads using the security policy when the JVM is started...
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And that would include filesystem, sockets, etc... http://java.sun.com/javase/6/docs/technotes/guides/security/permissions.html#RuntimePermission