Apple team has just posted iOS 4.1 in their Open Source category.
The iOS 4.1 Open Source is available under the GPL license, so you must read all the terms and conditions before using their code for commercial purpose. If you’re a new iOS developer, this Open Source version will be very much helpful to learn the iOS structure.
Apple has also released iOS 4.3 for users few days back. You can download it from here.
Download iOS 4.1 Open Source Files
[Thanks to those who sent this in.]