Apple has released iOS 4.1 Beta 2 Developer only version. You can download this beta version directly from the iPhone Developer Site.
There are reports that this build apparently solves the proximity sensor issue which was hindering some iPhone 4 performance. And there is a probability that this build will also solve the speed and lag issue on earlier iPhone models running iOS 4.x. The build number of iOS 4.2 Beta 2 is 8B5091b.
It is advised to not to upgrade to this build in case you depend on a carrier unlock as this firmware version upgrades your phone’s baseband.
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