The [[xref:http://www.pcworld.com/article/199410/apple_ios_4_complete_review.html|iPhone OS 4 upgrade]], or iOS 4, arrived Monday at 1 p.m. Eastern/10 a.m. Pacific for all iPhone and iPod Touch users who had their devices hooked to iTunes. The update weighs in at 378MB, and you need the latest version of iTunes (9.2) to be able to download iOS 4.
Unfortunately, if you have an original iPhone or iPod Touch, your best bet to run iOS 4 is to buy a new iPhone. iOS 4 works with iPhone 3GS, iPhone 3G, and second- and third-generation iPod Touch devices. Take note, however, that not all of its features are compatible with all devices. For example, multitasking is available only with [[xref:http://www.pcworld.com/reviews/product/545030/review/32gb_iphone_4.html|iPhone 4]] and iPhone 3GS.
Here's a closer look at some of the highlights of the latest version of the Apple iPhone OS.