Some notes on reading individual layers out of a Qgis Project file. This should end up in a 'import layers from a project file' plugin.
1. get the xml into some text:
xml = file("project.qgs").read()
2. make a blank QDomDocument and set the content:
d = PyQt4.QtXml.QDomDocument()
3. Now find the maplayers:
maps = d.elementsByTagName("maplayer")
4. How many?
5. Now read the fourth layer (0-index) into running Qgis:
Wrap this in a nice gui that tells the user what the map layers are, lets them check boxes, import the layers, marks the current project as dirty. Jobzagoodun.