This is a breakdown of Plone usage in different industries based on data reported at Plone.net.
There are many Plone sites that I am aware of that are not reported at Plone.net (I'm guilty, I don't list the Plone sites I build at Plone.net). Just visit delicious.com and check the plone-site tag. In this context "reported" means, someone responsible for the site visited Plone.net, and entered data, including a screenshot, description and link to their new Plone site.
I'm not surprised that the non-profit (NGO) sector tops the listings, I suspect that "reporting" is part of the non-profit "culture". Plone.net, being a self reporting system, is likely to show an over representation of those who are inclined to report.
As an additional note, I decided to only show industries that had a greater than 2% representation.
Plone Sites by Industry (according to Plone.net August 19, 2009)
Currently, no industry exceeds 20%. I left out some notables, such as Real Estate and Small Business which I believe are significant but were under 2%.
There is a great deal of overlap between News & Media and Publishing & Printing perhaps they should be combined.
Register with Plone.net Button
I know there's a quality/spam control issue with Plone.net, but it would be interesting if every plone site had a "register with Plone.net" button.