you may want to take a peek over this thread if you have not yet:
http://community.glowforge.com/t/how-to-determine-if-plastic-x-is-safe-to-use/648
this link is in the thread, and posted elsewhere here on the forum as well, but I’ll drop it anyway:
-a nice list of materials to use and not to use from a maker space in texas-
http://atxhackerspace.org/wiki/Laser_Cutter_Materials