The fourth generation, iPhone, dubbed the iPhone HD will likely land on Tuesday June 22, based on the company's reservations at the conference center where it has announced iPhones for the past three years.  The updated phone is expected to bring multitasking at last to Apple's customers.  A CDMA version of the iPhone also is reportedly in the works and may be announced as well, though it is not expected to launch in the U.S. until early next year.

Apple's iPhone, available (officially) exclusively on the AT&T network in the U.S., is top end hardware in terms of features and packaging.  It falls just slightly short of the newest handsets of a couple of its competitors, such as HTC's Nexus One Android handset, in terms of pure processing power.  In terms of its application catalog, though, none of its competitors can come close to touching it.

There are 150,000+ iPhone-ready third-party apps available in Apple's App Store, while Android only has 30,000.  However, Android and webOS have a key advantage over Apple: the ability to multitask (Windows Phone 7 Series, another key competitor will likely not support multitasking at first when it airs late this year).

Multitasking has long been prohibited on the iPhone to save battery life, despite the fact that the operating system supports it.  As of the third generation iPhone OS, multitasking has only been allowed in a few select apps, most of which perform critical phone functions.  

According to AppleInsider, a leaks site who first broke the news of multitasking in the fourth gen iPhone OS, sources close to Apple say multitasking on the new OS is fully enabled in pre-release builds.  They say the OS will utilize an interface similar to Exposé on the Mac (similar to Windows+Tab in Windows 7) to switch between apps.  Instead of resized windows, though, you'll get a series of scrollable icons for running apps, reportedly when you double click the home button.  That makes it sound closer to the Command+tab key combo found on Macs.  However, the Apple sources promised other aspects of the interface are very Exposé-like.

The sources also revealed two other improvements that may or may not make the final cut for the OS 4.0 release.  The first is the ability to drop shortcut buttons on the home screen to call frequent contacts.  For example you could add a "call mom" or "call girlfriend" button to your home screen.

Another update in a pre-release build is a global mailbox view, which would combine multiple email accounts into a common mailbox.

According to Jon Gruber, a nice hardware update is planned as well.  To counter the higher resolutions on Android and Windows Phone 7 Series handsets, the resolution reportedly will get the bump to 960x640.  Apple recently aired its new iFrame 960x540 video format, exactly 100 pixels less in width, lending credence to this rumor (the extra pixels would likely be reserved for important functions).

The CPU is expected to be in the Apple A4 family of systems on a chip (SOCs), which is thought to contain a Cortex-A8 CPU core (Samsung) paired with a PowerVR SGX (Imagination Technologies) graphics processor (GPU).  Apple's current iPhone 3GS uses an 0.833 GHz A8 chip underclocked to 600 MHz, while the new iPad is thought to use a fully clocked A8.  The iPhone HD will likely employ an A4, with an A8 computing core clocked higher than that in the iPhone 3GS, possibly running at 0.8 GHz or full speed.  In addition, the SGX GPU should be more powerful, by virtue than the iPhone 3GS's PowerVR MBX Lite GPU (which is currently on a separate chip).

The new operating system is expected to be announced at Apple's annual Worldwide Developers Conference (WWDC).  Apple reportedly reserved the Moscone Center in San Francisco, its typical conference location, from June 23 until June 24 for a "corporate event".  The event date was previously listed as June 28 to July 2, but it has subsequently been bumped to earlier in June.  While the center is not listed as reserved for June 22, that's a Tuesday, and every past iPhone launch has been on a Tuesday, so we expect that to be the true launch date.