James Governor ("redmonk") writes Seven Things About Computer Associates and Open Source Software with prominently at number two: plone:

CA was a driving force behind the formation of the Plone Foundation, which provides legal, organizational, and technical support for the Plone content management framework, built on top of the Zope server.

Yep, there's a big name behind plone, and from what I saw document-wise of the plone foundation in a really great community-oriented and open source way.