iPhone OS 3.1.3 may release very soon and is expected to be a minor firmware update and bug fixes but on the other hand iPhone Firmware 4.0 would be a major release and is expected to be shipped with the next-gen iPhone the coming Summer.
There is no info on iPhone OS 4.0 features but it’ll surely brought us more features just like iPhone firmware 3.0 did.
Update 1: iPhone firmware 4.0 beta 3 is out now:
Update 2: Orientation Lock and iPod Widget found in iPhone 4 beta 3
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