Apple released iPhone OS 4.0 beta firmware for iPhone 3GS, iPhone 3G, iPod touch 3rd generation, and iPod touch 2nd generation users. The build # is 8A230m. This firmware cannot be jail-broken, so you should not update it, if your phone needs jail-breaking.
If you want to jailbreak iPhone 4.0 or jailbreak iPod touch 4.0, please do not download any beta files as they may make it harder or even impossible for you to be able to jailbreak.
Since it has not been officially released, only developers can download it.
iPhone OS 4.0 beta download links
You must be a developer to download. Go to http://developer.apple.com/iphone/index.action#prerelease to download iPhone OS 4.0 beta.
