Can I access Datastore entities of my other Google App Engine Applications
Solution 1:
No, this cannot be done. However, as Nick Johnson points out, you can use remote_api to do what you need.
Solution 2:
Are you sure you really need to do this? Don't forget, you can have multiple versions of an application running against the same datastore. Only 1 version of the app is your "default" and gets your non appspot.com domain name, but you can have completely different codebases running against the same datastore/memcache addressable with ..appspot.com
I don't know if this satisfies your needs but thought I'd throw it out there.