Firstly, I understand how I can use google apps to point a domain (my company's domain say 'greatapp.com' at 'greatapp.appspot.com' no problems there.
I also understand that once I've got it onto 'greatapp.com' I can use wildcard hostnames to host the app on '*.greatapp.com' no issues there.
The point is that the application I'm thinking about building would be one which I would charge companies to use, and it would probably need to be co-branded, and therefore the company might want to use their own domains for displaying their data to customers, so data.companyname.com instead of companyname.greatapp.com.
Would GAE accept a domain pointed by the company via CNAME (or whatever) or is that not possible?