We have no "phone-home" feature so there is no definitive answer to that. While most public cloud providers publicize the software they used, most private clouds (except in semi-public areas like government or academia) are generally a lot more shy.

The data from the User Survey, as well as the User Stories on the website, give some examples. Others can be derived from the long list of companies supporting OpenStack.

Slide No12 in the presentation http://www.slideshare.net/manthang/open-stack-101vnmeetup3manthang

There are also some reference user stories at http://www.openstack.org/user-stories/ .

We will be publishing additional data at the openstack summit in November.

http://personal.crocodoc.com/4YprKmn - there are over 151 cloud operators out there according to the user survey, and 197 reported deployments.