B2's and F117's don't show up on primary radar... First time I actually had one on frequency in my sector, I asked each to squawk standby... no primary targets, way cool. Last month had a Gulfstream with a failed transponder in RVSM airspace, FL400.... Invisible airplane... (we don't normally look at primary radar at high alt sectors). Thank God for the "Big Sky Theory" and the back up transponder onboard the aircraft.