App Engine apps don't exist on any one serve, they are distributed
across google's architecture. Accordingly they don't have a single IP
address. Additionally multiple AppEngine apps will frequently run on
the same server, this means that using an IP address to authenticate
an App Engine app is highly insecure.
In regard to the later DNS question, according to the App Engine docs
(
http://code.google.com/appengine/articles/domains.html) the way you
use a custom domain name with App Engine is through google apps for
domains. Basically you link you app engine account to your google
apps for domains account, then you set google's DNS servers as the DNS
servers for your domain. This means you don't need to know the ip
address of your App Engine app and your not limited by the
capabilities of your registrar's DNS servers.