The Administration console doc notes that if you have a Google Apps account, you need to go the following URL:
http://appengine.google.com/a/your-domain.com
Please replace the relevant parts of the above URL.
It also notes that you can upload your apps using appcfg.py in the SDK.
Speaking of the SDK, the app engine SDK includes a web server that simulates the app engine python environment. More here.
The python runtime is restricted, which is why you should test your code with the app engine SDK. Some modules such as marshal, imp, ftplib, select, socket are empty. More here.
Logging is allowed, and can be viewed in the Administration console.
Google App Engine looks to be tweaked for running web applications only. Application code only runs in response to a web request. This is unlike Amazon EC2 where you can build/upload an image for data processing. If a request takes too long (this is vague, it says more than a few seconds) its terminated and the server returns an error code. Using the Datastore, you have 500 MB of persistent storage space. You also have bandwidth for about 5 million page views.
Python extensions in C are not supported.
During the preview period you can register up to 3 applications.
Currently a query is limited to a 1000 results.
Here is the official Google video on the release:
