OK didn't know about .xcf but the problem is. The format is neither used in paint.net nor in Photoshop. Which are the two programs used by my customer.
Well .psd could be used, as the two programs support this, but for archiving I would prefer an open file format. And GIMP only for converting .psd into .xcf is not really practical.
Well I will have to wait might be the OpenRaster format will be better adopted.