Ning co-founder Marc Andreessen wrote a now famous (famous in SiValley at least) blog about the three kinds of platforms on the Internet. Andreessen claimed that Ning was the only Level 3 platform, where your code runs directly on the platform. Clearly the Google App Engine is also a Level 3 platform.
Ning and App Engine seem different, and they are. Ning has built an application called the Ning Social Network, which is an application that runs on the Ning platform. When you use Ning, you get the code for your own instance of the Social Network and then customize it from there. Can you rip it all out completely and build something from scratch? I don't know. But essentially that is what Google gives you: Ning with non Social Network app.
But wait, what about BigTable? Ning has a little thing called its content store which lets you store your own data structures and then query them ... a lot like BigTable. I am no expert on either, but any Level 3 platform has a similar need that is satisfied by these technologies.
Of course the other big difference between Ning and App Engine is that Ning is PHP based and App Engine is Python. That is also a major difference they both have from the AWS collection. They both support a single language with a set of advanced platform APIs implemented in that language. The other big difference is that both Ning and App Engine are free.
Personally I am thinking of writing a generic web proxy to deploy on App Engine. This would be useful for any Flash/Silverlight development to allow scripting to a site that does not have a nice cross-domain policy, such as ... Ning and Google :-)