Here's a feature I would like on my mobile phone: the ability to "ping" someone with 2 or less keypresses (something that takes around a second to do), even if the keypad is locked. The idea is that when I press a couple of buttons on my phone (say, 1#), a tiny content-less message is sent to the person corresponding to that key combination. On their end, they see something like "Jason pinged you at 7:34pm" with the option to ping right back. You'd have to set up what pings mean beforehand, stuff like "I'm leaving work now" or "remember to pick up milk at the store".
Pings would be perfect for situations when texting or a phone call is too time consuming, distracting, or takes you out of the flow of your present experience. If you call your husband on the way home from work every night and say the same thing each time, perhaps a ping would be better...you wouldn't have to call and your husband wouldn't have to stop what he was doing to answer the phone. You could even call it the "sweetheart ping" or "sweethearting"...in the absence of a prearraged "ping me when you're leaving", you could ping someone to let them know you're thinking about them.
This reminds me a bit of Matt Webb's Glancing project: I'm Ok, you're Ok. I guess you could think of pinging as eye contact via mobile phone...just enough information is conveyed to be useful, but not so much that it disrupts what you're already doing. Webb cites Howard Rheingold's Smart Mobs:
Howard Rheingold in his book Smart Mobs gives a good example of text messaging being used for this. He talked about kids in Sweden after a party. Say you've seen someone you quite liked and you'd like to see them again, but don't know if the feeling's shared. You'd send them a blank text message, or maybe just a really bland one like "hey, good party". If they reply, ask for a date. The first message is almost entirely expressive communication: tentative, deniable.
Matt does a fine job explaining why this stripped-down style of communication is sometimes preferable to more robust alternatives.