codlink:
Did you make sure that your CAM file had that layer highlighted when you generated the Gerbers? I did that once..
Fwiw, if you use Eagle, iteadstudio is nice enough to publish a DRU File to ensure that your design is good to go, as well as a CAM processor file to ensure that the proper layers are printed/processed. Now if only eagle was easier to use (I find the cam process completely backwards, for example).
After 'printing' your Gerber files, the online Gerber viewer will let you see exactly what will be transmitted to the pcb fabrication house.