Is it possible for a server running on Google App Engine to receive an RTSP stream?
If so, how easy is it?
Yes and no, depending on what you are trying to achieve.
RTSP is only a control protocol, the streaming part (delivery of media data) is done via RTP, which is UDP based protocol and hence not supported on App Engine.
RTSP itself has a similar format to HTTP so you could implement it on GAE. For example in Java you would extend GenericServlet and handle custom RTSP verbs.
So, you could possibly do the control part but not the media delivery.