Seth Gottlieb has posted a good Alfresco overview with a brief comparison to Plone. Seth concludes,

I would use Alfresco for a targeted document management solution that would fit into a larger enterprise content management architecture…I would use Plone to build an all-in-one intranet or extranet where I wanted to mix article, page, and file content and opportunistically deploy new features to improve collaboration and retention.

I agree with Seth’s conclusion and the point he makes in the article that the lack of LDAP integration in the community edition of Alfresco limits it to departmental use. I’m sure they’ll move that feature down at some point–when they do it should boost adoption.