Apple today announced a technical preview (for registered developers) of iPhone OS v4, which will be released to the public this summer.

It adds a whole slew of new features and capabilities, one of few of which will apply only to the latest models of the iPhone due to computing power requirements:

iPhone OS 4 will work with iPhone 3G, iPhone 3GS, and the second- and third-generation iPod touch this summer, and with iPad in the fall. Not all features are compatible with all devices. For example, multitasking is available only with iPhone 3GS and the third-generation iPod touch (32GB and 64GB models from late 2009).

The mail and multitasking capabilities will be big, so will the addition of eBooks (using the same store as the iPad) and the ability to organize apps on the screen into folders. Today you have to page through screen after screen, all on one “level,” but with the new OS that story changes.

At the enterprise level, there are some other substantial changes and improvements in areas such as app distribution (wireless app distribution for the enterprise is here) as well as security and device management – and all of these are important if Apple wants to make the iPhone something a secure and well-managed enterprise can leverage. Cisco and Juniper are working on SSL VPN apps to allow secure access to the enterprise, as well.

Oh, and they are adding their own advertising network to the iPhone app story, with a service called – shockingly – iAd. Go figure.

The iPad will get OS v4 this fall, according to Steve Jobs during a press Q&A session.

Too bad I don’t have a 3GS, so no multitasking for me until i replace this iPhone (which now sports a badly-cracked glass screen, but still works great).