There are two recent articles on the state of OS CMS: "The State of Open Source CMS" by Gregor Rothfuss in the Cutter IT Journal which you need to subscribe to read; and "Straight from the Source: Open Source Content Management" by John Harney in inaugural issue of Enterprise Open Source Journal.
Writing on this topic is a daunting task because the space is so expansive and dynamic. These authors have both done an excellent job of introducing the "market" and describing some of the trends that are happening.
Gregor Rothfuss is very qualified to write on this topic. Not only is he a committer on the Apache Lenya project, but he is also an officer of OSCOM - the organization of open source CMS projects. One of Gregor's more interesting points is that the OSCMS space is overcoming a history of fragmentation with no clear market leader. One of the developments that may change this is the Java Content Repository (JSR 170) which will lower switching costs and allow different CMS to become more interoperable. Open source CMS projects are outpacing their proprietary counterparts in adopting the JCR partly because vendor lock-in is not on their agenda.
John Harney's article is a good introduction to content management and open source. In it, John discusses base content management features that are supported by open source CMS and the advantages and disadvantages of adopting an open source solution. There is particular attention paid to Zope with mentions of OpenCMS, Mambo, TYPO3 and WebGUI.