AFAIK, they have a portlet that does what you want
There is a portlet that show the whole Alfresco application (within one portlet window) : to manage the user, create content etc...
If you want to show one content, like in the CMSPortlet, you have to adapt a CMSPortlet that access the Alfresco CMS repository.
Access to this repository instead of the JackRabbit one, that is the one set by default (in 2.4... 2.6 I don't know yet what is set/proposed...).
there are several ways to do this, but quiet a lot of work as integration.
there are previous post about this.
version 2.6 may propose this feature (a CMSPortlet that work with Alfresco repository).
look at post from author firstname.lastname@example.org, that said he is working on this.
on my own, I wait for this to go further with Alfresco. Too much work if I must do it from scratch.
hope that infos helps you...