When I open a QGIS file, QGIS 1.8 resets the scale factors for all labels. Is there a way to save the file so that it knows to also save the label scales?
It is strange that, when you save a project, it does not keep the style of the labels as well. I think aynthing will be better once the (now development) 1.9 version of QGIS will released with the new labelling system. (You can of course install the dev version from the OSGEO installer, I use it daily and I have no stability problem at all)
I would anyway suggest you that, when you are finished with the style editing of a shapefile, to save the style of the shapefile as the "deafult style":
in the style tab of the shapefile "style properties", at the bottom "save as default"
This creates a style file (.qml) assosiated to your shapefile which stores also the label formatting.
In this way anytime you import that shapefile in a project it keeps the style format and the labelling. hope it helps