Depends on what they're riding.
If I'm on my motorbike, I don't nod at people on mopeds unless they nod first. In my opinion they probably don't know about the nod, so would be pointless. Maybe 50% of my nods are returned from other riders, less in town riding, maybe because they're just busy on other things, more nods out on country roads.
When riding the road bike, anyone who looks like a cyclist, either commuting, club runner, or lycra clad roadie, or around-town lady doing her shopping on her sensible bike (we have a lot in Horsham) will get a nod, or a "morning". People riding junkie looking bikes that they probably just nicked won't, unless they nod or talk first. Again probably for the same reasons as moped-crew. Don't know the etiquette required for their given mode of transport.
If I'm on my junkie around-town mountain bike and any "proper" cyclist overtakes me, I always say my other bike is a roadie!!! But don't expect any greeting back - I look like a mtb peasant!!!
I also always stop if somebody looks like they might have broken down, just today a couple of guys were stopped at the side of the road, one of them spinning their front wheel in a suspicious fashion, etiquette dictates one most stop and make sure they don't need a spare tube, chain link, chain tool, allen key, patches, CO2 carts or any other help.