Martyn Taylor and Steven Hardy have done an awesome job of demoing an Image Building Service for OpenStack:
http://www.youtube.com/watch?v=MdBM4HA3QUk
I think this has huge potential. Imagine an OpenStack API to which you could send a request for a fresh image build of any OS, request specific packages, software or other content to be included and have that image be uploaded to Glance or Cinder once it's built. The image gets built in the cloud on a Nova instance/VM and the cloud provider bills you for the compute and I/O resources needed to complete the build.