Exchanges like the one on the right are why I'm really, really wishing the iPhone had a built-in chat application (not just SMS) that could run in the background, even though I don't own an iPhone myself. (I've highlighted the time stamps to make it obvious how little time I had to respond before Marissa dropped offline.) Because folks are using an installed, application to reach their IM logins (most likely Google's own Google Talk app), they're not able to stay connected unless that application is in the foreground, because Apple doesn't allow third-party applications to run in the background. So you get that contant login-message-logout pattern from iPhone users. In an ideal world, Apple would let third-party apps run in the background, but on a mobile device I can understand why they don't. So they should at least provide a "built-in" chat application that can connect to the major IM networks and stay logged in even when in the background, given that instant messaging is as common a method of real-time interaction these days as voice calls or SMS (at least for certain people.)
If Apple is just doing this to force folks to SMS via the AT&T network, they should be ashamed of themselves. There's no reason users should have to be paying on a per-message basis for sending 140 character text clips when they have a TCP/IP connection available for web browsing, etc on an unmetered basis.
Even my Blackberry Curve handles this issue better than the iPhone -- and it's pretty rare that I say that for anything unrelated to its hardware keyboard.