Welcome. Reading it made me hate my rat-race life even more...!
It's interesting that you find that to be the case. I have noticed a trend of fewer riders going to the effort of acknowledging each other these days, which is a shame. It's always a nod in my neck of the woods, but a few wave and you can tell the riders from the continent when they stick a leg out
I try not to examine too closely the likelyhood of another rider ignoring my nod based on what they're riding, but I can't help but notice that it does seem that adventure bike riders seem less likely to nod/wave than most, GS riders in particular. No offence to any potential BMW owners present, but in my experience it just seems a "you're not in the club" kinda thing. Not all of them of course, but seemingly a higher percentage than the average.
Interestingly, cruiser/chopper/Harley riders always seem happy to acknowledge me, regardless of the fact that all I've ever ridden have been Japanese bikes (in fact - all I've ever owned have been Hondas!). Puts the whole "cruiser-riders hate sports-bike riders" thing in perspective, but perhaps that's just an american stereotype. Any of you guys from the states find that to be the case?