I'm with a little problem! I have a map with some layers! everything seens to work well, include the labels.
In my MXD file, I have configured to hide labels if the scale is out beyond 1:1.500 (Layer properties / Labels / Scale Range / Don't show labels when zoomed / Out beyond 1:1.500). In the desktop, everything worked well! Tested on scale 1:2000 and the label don't show! on scale 1:1000 show.
Then, I created a silverlight application, and load the map dynamically: