The following are four significant impacts I believe social publishing will have on the CMS market.

1. Content management systems will become a niche application. Over the past couple years, I have loosely defined forum applications (SMF, phpBB), wiki applications (mediaWiki), and blogging applications (WordPress) as niche or subsystems of the Web CMS. Under Whatcott's model, content management now becomes a pattern or subsystem of the SPS.
2. The best Web CMS of today will need to evolve into social publishing systems to survive. It's not just Drupal that is repositioning its focus on social publishing! Have you also taken a look at what SharePoint and Wordpress are doing lately?
3. The rise of SPS will hasten the consolidation of CMS products in both open source and propriety markets. See Dries Buytaert's bold statements under the header, Market predictions for 2008. Buytaert discusses the big three CMS that he sees surviving and those that will struggle under consolidation (I would shuffle the list a little).
4. Social publishing systems will be designed for more than just the desktop browser. What good is social software, if the system isn't also accessible on your PDA or SmartPhone? Hasn't Apple's iPhone shown us that people want to do more than just voice and text messaging on their phone?

If you really think about it, the purpose of the Internet was not to manage content. Instead the Internet's purpose was to exchange information, whether this is from computer to computers or person to people. In other words, content management systems really only meet a part of our needs for why we use the Internet. The social publishing model, with the scope and agility it is designed for, seems to be ready to do what the content management system was never designed to do. I think social publishing is here to stay and all that needs to be done is to spread the word.