Apple has released iOS 4 for the second- and third-generation iPod touch, iPhone 3G, and iPhone 3GS. The fourth major release of Apple’s mobile device operating system—previously called iPhone OS—the new software provides over 100 new features or enhancements, according to Apple. Among these are multitasking, folders, customizable backgrounds for the Home screen, a unified inbox for the Mail application, more fine-grained control over Location Services, support for Apple’s new iAd mobile advertising platform, and support for iBooks 1.1 on the iPhone and iPod touch. Several of the features—most notably multitasking—require more advanced hardware and are therefore only available on the iPhone 3GS and third-generation iPod touch, as well as future devices such as the iPhone 4. Interestingly, iLounge editors have seen a dialog box experienced when updating a second-generation iPod touch, warning not to “interrupt the update, which may take an hour or longer to complete” that was not seen when upgrading an iPhone 3GS. The iOS 4 update for the second-generation iPod touch weighs in at 315.0MB, the iPhone 3G update is 292.1MB, the third-generation iPod touch download is 366.4MB, and the iPhone 3GS version is 378.0MB. iOS 4 for the iPhone and iPod touch is available for download now via the update feature in iTunes, and will be coming to the iPad this fall.