We've gotten a few reports that Apple has released a minor build update for the new Apple TV. Users aren't reporting any obvious new features or specific performance upgrades with the brand new 4.2.1 update, so it's likely that this latest Apple TV update is just for bug fixes. Not all users are seeing the update quite yet, but it should propagate to everybody before long.
MacRumors notes that the Apple TV iOS 4.2.1 update is labeled Build 8C154. The previous Apple TV iOS 4.2 was Build 8C150. If you Apple TV owners see any differences, let us know in the comments.
