Not sure if anything like this already exists either. It well might.
That's kind of a huge thing - it's the difference between this idea working and not working on the iPhone... and a great example of the kind of app innovation Apple could inspire by allowing background processes.
They just need to expose specific interfaces to launchd (Apple's clever init/rc/cron/xinet replacement), for doing particular kinds of local notifications. An app could register (with user confirmation) that it wants to have a limited helper run when an event has occurred: cron-style schedules, at-style timers, "Moved more than X meters", "Am near X:Y", etc. The registrations would be managed just like with the current push notifications.
With that, there'd be only a few other APIs needed to absolve the want for background processes. The ability to enqueue HTTP requests would make quite a number of apps friendlier to multitasking, and its already implemented privately in MobileSafari. The ability to play audio in the background would be huge, especially combined with queued HTTP requests, but I suspect that it'd be a lot harder to squeeze out of Apple.
I guess if you fellate / pay Apple enough, they let you do it.
It does appear to only cover one of his use-cases, sadly. But the others would be difficult without backgrounding (and the theft alarm may require far too much polling and battery drain to ever be useful.)
A game where you try and assassinate your friends by setting imaginary time bombs to go off at a particular location when you think your friend will be there.
Of course it could also be built on top of something like Loopt or BrightKite if you and your friends compulsively check in everywhere you go.
Wait a minute... Firing up the Ycomb application right now ;p