John Gruber skims through the iPhone updated HIG (Human Interface Guidelines) and reveals the following:

Only one iPhone application can run at a time, and third-party applications never run in the background. This means that when users switch to another application, answer the phone, or check their email, the application they were using quits. It’s important to make sure that users do not experience any negative effects because of this reality. In other words, users should not feel that leaving your iPhone application and returning to it later is any more difficult than switching among applications on a computer.

Will there be a function to preserve state between App uses? (The way iTunes remembers what song you were playing and where you were in that song) Or will each App have to load fresh each time? (The way photos gives you the album chooser, rather than the last photo/state screen upon launch).

Treo users have been stymied for years by lack of true preemptive multitasking, while Windows Mobile has been criticized for letting Apps pile up like splattered bugs on an speeding windshield. But functionality like background music playing, downloads, data (i.e. email checking now that it's push) seem like no-brain must have'ems. What's the deal Apple? And what do you think?