I was wondering the same thing. I got my car only a month or so ago and have been waved to twice. I initiated the wave once and didn't get one back.
This reminds me of the "Jeep Wave". For years, Jeep drivers (Wranglers only) would wave when passing each other on the road because there really weren't that many Jeeps on the road. It was a special occasion and worthy of a wave. Now every other vehicle on the road seems like a Jeep or a Mustang. Needless to say the "Jeep Wave" had reached a point where it was getting silly to wave 10 times on my way to work. Challengers aren't nearly as common as some of the other muscle cars on the road and they really stick out. Shoot, a lot of people just stop in awe because they don't even know what it is they're looking at LOL. I had an older guy at a Brad Paisley concert tell me what a great restoration job I did and asked the year - you should have seen the look on his face when I said 2010. So yes, I think its more than appropriate to wave or nod to other Challengers on the road. We're a rare breed...for now.