Apple has just released the Beta 2 version of iPhone OS 4 for developers, along with an updated version of SDK for iPhone and iPod touch.
iPhone OS 4 Beta 2 and SDK includes new API improvements, features and bug fixes. Stay tuned, as we will have more on the change log soon.
Earlier this month, Apple unveiled the new iPhone OS 4 firmware in a preview event, held at their 1-Infinite loop campus in Cupertino. This announcement added lots of new features for both users and developers alike.
All the jailbreakers and unlockers are recommended to check out the updated jailbreaking notes on iPhone OS 4 posted here.