So I set out on a quest to find out if Redhat was within their rights to make such a bold claim.
I knew it would be hard, but I'm the "Storage and Servers" editor for Network Computing. If I couldn't figure this out and let you know, who could? So I hit the old research.
I found what has frustrated many of us for years is still very true. There is no good way to count Operating Systems installations, or for that matter any Open Source installation other than Web and Application servers that are public. For web and application servers, NetCraft does a fine job, even if some decry the methodology of only looking at public servers.
The problem is the same old one - that a single sale or download could represent a single machine, or it could represent an entire enterprise data center. Since enterprise software architectures are not something companies like to advertise - giving away information about your environment is in the I.T. Security Handbook under the heading "stupid" - so even polling is difficult.