It sounds like you're observing the laser etched glass focussing screen. A focusing screen requires a rough surface so an image can be formed for you to view through the viewfinder.
Ground glass used to be used for this purpose but now glass etched in a circular pattern with a laser is used, to improve the brightness of the viewfinder when used with slower lenses. This pattern is probably what you're interpreting as moire.
Here's what a standard DSLR focussing screen looks like close up:
http://www.mattgrum.com/experiments/focus_screen_closeup.jpg