Hello,
I just stumbled in obj exporter problem with selected meshes(it reported an error).
Just simply looking to api:
http://www.blender.org/modules/documentation/237PythonDoc/Blender-module.html
you need to substitute
line 564, in save_obj_ui
export_objects = Object.GetSelected() # Export Context
with
export_objects = Blender.Object.GetSelected()