iPhone OS 4 beta is available for registered developers.

What are we talking about

If you are a registered iPhone application developer, you can download and install firmware with a new version of the OS on your phone, without waiting for the summer release.

Supported Models

• Phone 3GS
• iPhone 3G
• iPod touch 3rd generation
• iPod touch 2nd generation

What happened

The phone works fine with iTunes and Xcode 3.2 (with the new SDK), all applications have experienced an upgrade. Inside there is everything that everyone has been talking about lately: “multitasking”, grouping applications, etc. I like the review of features from here - soft.compulenta.ru/522087
Bugs in the work has not yet been noticed. From the pleasant little things - the phone has ceased to forget the hidden Wi-Fi networks.

UPD : from the words of Slev7n from the comment “the update can be done only with MAC. and the term of the first beta will expire on June 2. ” About Mac did not pay attention, because updated from it. Download give the usual ipsw. Until June 2, I hope there will be more beta, and there, you see, and release.
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UPD2 : beta 2 came out on April 20th.