Is there any need to penetration test a GoogleAppEngine solution? Or will it be protected by googles existing infrastructure?
Just because Google hosts the site doesn't make it anymore secure.
It is still your code that is open to exploitation. If it contains business critical information or user data then it is prudent to use secure programming methods and other practice's to ensure the security of the system and your data. This could include penetration testing.
But be sure you are carrying out the testing against your own system and not Google's underlying infrastructure. Also be sure to check the terms and conditions for Google App Engine to ensure penetrating testing your site/app is not in breach of them.
Note: the above audit blog post doesn't mean insecure code hosted on Google's systems is guaranteed. It means their overall infrastructure has been audited and certified. Your code can still be vulnerable.