A couple of good posts this month on whether a single CMS can be used to manage both your public and intranet web sites. Short answer: usually, no. For more detail follow the links.
I agree with those authors that more often than not, a single CMS will not be appropriate for your public site and your intranet site. The requirements for each are going to be pretty different once you get past basic authoring and content management features. Even for associations, whose public web sites could be considered more intranet-like than the usual corporate web site, are going to be hard pressed to find a single tool that effectively supports both.
C. David Gammel, CAE 
The creator of the web framework Ruby on Rails made a provocative post in a similar spirit a few months ago. His core aphorism on the subject: “The more expensive it is to create fresh software, the more appealing the mirage of generalization will appear.”
Which is not to say that integration is not a good goal — it’s essential — but as the pain of making the right tool for each job decreases (thanks to frameworks like Rails, Django, et al.) so does the appeal of the One CMS To Rule Them All.
http://www.loudthinking.com/arc/000528.html
I could definitely see one organization using the same framework to develop and deploy very different public and intranet sites.