I don't read much into nods. However I always nod back on BM out of courteousy and respect to fellow members. It's the same on the road.
Earlier on in this thread, it was mentioned about only nodding if safe to do so. I cannot see many instances where it would be unsafe. but that depends on your style of nod. I noticed many bikers just give a quick sideway flick of the head, can't see a problem with that in terms of road safety.
The other week I had a nod and a thumbs up from a motorcycle cop while on a call, lights a flashing and sirens on. Mind you I think riding a classic bike does get more reaction from people hence more nods. Or are they thinking, look at that poor old sod lets cheer him up.