Thing is, you either get massive battery life degradation or your messages won't get through, there isn't much of a choice there unless you use push messaging. Google Cloud Messaging allows Signal's servers to send a "Hey, wake up and connect to us!" message to your phone, and every other app uses GCM for this as Google has spent a ton of time trying to optimize both reachability and battery life with GCM.

Maybe something will come along to supplant this, but for now push messaging is how we deal with terrible state table problems at the cell network operator level without wrecking battery life.

Not only that, Google apparently has agreements with mobile network operators to ensure that their long-lived connections don't get closed after a few seconds or minutes of inactivity. Some mobile ISPs close idle connections after as little as ten seconds, so unless you're in a position to negotiate with just about every major mobile network operator worldwide, you can't really offer the same level of service.

Apple likely has similar agreements too. I'm hoping that IPv6 will help alleviate many of these issues, as while your address may change as you move between towns, the state table needing to be maintained will be much less complex.