Just a bit more than a year after we first laid eyes on iPhone OS 3.0, Apple is back with the latest big revision of the OS that powers the iPhone, iPod touch and iPad. iPhone OS 4 is shipping this summer (iPad in the fall), and the developer preview will be out today. iPhone 3GS and new-gen iPod touch will get all the features, but some features won’t make it to the iPhone 3G, original iPhone, and older iPod touches. The biggest new feature is multitasking, which Apple says is going to be the “best” implementation in the smartphone space, though it’s obviously not the first. App switching is activated by double tapping the home button, which pulls up a “dock” of currently running apps, and Apple claims it can do this without hurting battery life or performance for the front app. Unfortunately, this multitasking won’t be available for devices older than the 3GS and new iPod touch. Multitasking is just one of seven different new “tentpole” features, including Game Center, enhanced Mail, and more… More >