Google has released a Google Talk client for the iPhone that allows instant messaging as long as the application is open. I’d like to think of this as a nifty way around rising texting costs, but that’s unlikely, given how much time my phone spends in my pocket. If this type of mobile app takes off, it will raise a usability question for the high-end phone and MID apps developers. So much of our PC lives revolve around multiple applications staying open — and around the user focusing on the machine — but that isn’t how people use their mobile devices. So how do you build a phone that allows for multiple programs to be open, and how do you alert users to changes in the app’s status without going through carriers?