Embedding layers into QGIS projects
Marco Hugentobler has implemented the ability to embed layers and groups from other QGIS projects into a current project (available in both QGIS and QGIS MapServer). This can help eliminate the extra work of “laying out” data in a TOC when the same data is used in multiple projects. Simply go to the “Layer → Embed layers and groups” menu, select the source project, and select the desired layers/groups.
Embedding does not involve copying data but using links that can be either absolute or relative (depending on the project settings). Accordingly, any changes made in the source project will be reflected in the target project.
If you need “real” embedding, the ImportLayersFromProject plugin (by Barry Rowlingson) written the day before comes to the rescue. The plugin also analyses the source project and allows you to transfer a layer completely from one project to another. This eliminates the dependency on the source project, which can be modified or even deleted without losing the data embedded in the target project.