I think its depron... found a link to a plane in the same series with a higher resolution photo:
http://www.rcgroups.com/forums/showthread.php?t=1012493
Nice shiny foam... methinks depron.
But combat is a great idea and I'm suprised we haven't taken it up yet. Maybe we should settle on a power train, pick a depron design and have another craft night.
I'm thinking 10g motor, 10A ESC, 2S 500mAh lipo, 2x 5g HXT servos. That keeps it light and small that we can make them out of 3mm depron. This also makes it ultra cheap to cut a new airframe when you crash and burn and my surviving aircraft does victory rolls over your crumpled pile of depron.
This could even make a cool FT show, eh?