Yeah .. It's weird, The Psychology of Bumming (c) (I can see Norvig's future "Teach Yourself How To Bum in Ten Years" or ESR's "How to Become a Bummer").
But in all seriousness, I get your point: It's not that you want their money, just want the gesture.
It happened I paid for their coffee, or gave a gum. Or proposed to buy it (for half of the pack's price). Reciprocity.
On another note, there's an amazing book by Robert Cialdini called "Influence: Science and Practice" (the old one, not the new) which should appeal to hackers: It's about hacking people's brain.
It is the greatest book I've read on the matter: All empirical data.
In all other cases I much prefer when people offer to pay for a cigarette, even though I've never taken their money.