Grab it now if you're a developer. There doesn't seem to be anything noticeably new in the documentation, other than some API changes. We'll have to see if this includes the "fixed" reception bar display and proximity sensor tweaks. [Apple]
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I'm wondering if the new notification system they are hard at work on will appear as a 4.x, or will they wait until next year for 5.0?
Please 4.x, please?