How would that be possible you ask? Well, a couple things are required.
The Repoze guys have Plone running as a WSGI app, AFAICT. So the hosting part is pretty much settled. You just deploy an app that sets up a Repoze WSGI App.
Then comes the storage. Plone, being based on Zope, uses the ZODB. But the Google App Engine doesn’t allow writing to the filesystem. The solution? Having a RelStorage backend that uses the DataStore API.
The final step would be getting rid of the C extensions in Zope, and having Zope run in Pure Python. Fortunately, there’s a student application for this year’s Google Summer of Code project that intends to port Zope to Jython! That should help with this last step by providing Pure Python implementations of the C extensions, if it gets approved.
I am really excited by the possibilities that this opens up. And would love to see someone pick up this project and turn it into reality.