The hard problem on iOS devices (iPod touch) was keeping them awake during this, so they could detect eac other. iOS likes to sleep while locked. Got a solution, which made the battery last around a day. I guess they're not using mobile to mobile peer triggers, but fixed-to-mobile and then deliver a notification. Interesting to see this popping up in commercial tech.
I was going to spin my code out into a infrastructure for spatial awareness triggers for various home automation tasks, but also couldn't find a killer use case. I'll be following this topic for ideas :)
I use it in my app OnSite Time Tracker.
I've written a similar (open-source) app called Remindly (https://github.com/nathanstitt/remindly) that uses region monitoring to fire alarms.
I've found it's region monitoring accuracy to be horrid - either the alarms fire when I haven't moved, don't fire at all, or fire an hour later when I'm 20 miles outside the region.
Since I have the same issues with the built-in Reminders app, I suspect it's something specific to my geographical area, or the api is just rubbish.
Putting features like that into other apps would be fantastic.
I don't mean to turn this into an, "Isn't Tasker great?" rave, but... isn't it?