Surf is the new framework with Alfresco 3.0. Surf is backed by Alfresco's web scripts. These are REST services. Thus, it is possible to mash-up with any other system supporting REST based services.
Alfresco repository implements Microsoft Office SharePoint protocol so it is possible to use MS Office with Alfresco the same way you can use MS Office with Sharepoint.
So if you are looking to mash-up Alfresco Share client with content from another app, you need to REST enable your back office system. There are tools and frameworks that work well with legacy apps.
Hope that helps.