From this question I learned that Google App Engine does not currently support SSL on \"custom domains\" (at least not as of June 2010, when that question was asked).
<(As the others on here have said) SSL is not currently supported for your own domain. It is aparently on it's way but has been for some time, I believe it is currently only available to a select few Google App Engine for Business customers.
The temporary solution which many (myself included) are using is to setup a reverse proxy from another hosting service (Amazon EC2 in my case) to route SSL traffic.
If your app suits the situation where your URLs are not of importance, you could setup an SSL site somewhere and access your https://xxx.appspot.com version from within an iframe
Either way until GAE offically supports SSL via your Google Apps domains, you will need an external service to workaround it.
Priority:
It is at the top of the Features on Deck list.
http://code.google.com/appengine/docs/roadmap.html
Simultaneous serving:
A custom domain hosted app such as http://www.mydomain.com can still be accessed on its ssl appspot subdomain such as https://yourapp.appspot.com
Issue:
http://groups.google.com/group/google-appengine/browse_thread/thread/844dc97fbfc57bab/0c8651f00072f9ea?lnk=gst&q=ssl#0c8651f00072f9ea
Custom SSL is available for App Engine since 27 Jun 2012. You can setup it from your domain's control panel: https://developers.google.com/appengine/docs/ssl