I think that the vast majority of applications can be designed without exit buttons. Safari on the iPhone is a great example (I'm using iPhone applications bec

I think that the vast majority of applications can be designed without exit buttons. Safari on the iPhone is a great example (I'm using iPhone applications because I'm more familiar with their behavior). Safari always keeps track of its state on disk. When it's closed, if pages are open it pauses some tasks (e.g. JavaScript timers) but keeps running unless it gets a low memory warning. If no pages are open, it exits completely. The user doesn't think about closing Safari, he thinks about closing web pages. Mail on the iPhone stays running if the user has set it to check mail in the background. The same principles can be applied to applications on any mobile OS. Pandora should stay open if music is playing. A Twitter client should stay open if the user has set it to check for Tweets in the background. An IRC client should stay open if there are active connections.