Got quite a few emails in response to my post on sweethearting/pinging. Several people mentioned pranking[1] as a current implementation of this idea, a trick I remember using as a kid. You call someone and hang up after one ring..."prank me when you're outside my apartment and I'll come down". Pranking is typically driven by economics...you don't pay for a phone call that doesn't connect.
Gen Kanai asks: "why can't SMS do this?" It certainly can; if I were implementing sweethearting, I would piggyback it on SMS. But what I'm really concerned with (as usual) is the user experience. To send a blank text message to a specific recipient with my phone takes at least 6-10 keystrokes. I want to do it in two keystrokes and (in time) without looking.
[1] I received reports of pranking being used all over the world. It's called one-belling (or pranking) in England, people send "toques" (roughly "touches") or "sting" each other in Spain, Italians "fare uno squillo" (which Google translates as "to make one blast"), and in Finland it's called "bombing".
Update: In South Africa, they call it a "Scotch call".