I think LPH has it right--we acknowledge people in our sub-group, and for better or for worse, there are sub-groups of the overall cycling sub-group! I've had the same experience where people nod, smile, or wave when I'm on my road bike (funniest reaction is when someone STARTS to acknowledge me, sees the fenders and rack, and realizes that maybe I'm NOT in their sub-group and turns the acknowledgement into an awkward I-meant-to-do-that gesture) but don't even seem to see me when I'm on my Townie with a purse in the front basket. The moms with strollers, however, DO acknowledge me on that bike where they don't on the road bike.

Sarah