Apple just released iOS 4.1 Beta 2 to developers, and unless you are a dev, you have absolutely no business downloading and installing it on your iPhone.
If you do, make sure you don't care about future jailbreaks and unlocks because you might lose your ability to do it.
There doesn't seem to be much in this beta release except for the proximity sensor fix, which hopefully will prevent those affected from randomly hanging up on people.
The iOS SDK 4.1 beta was just seeded to developers a few hours ago and believe it or not, you can already jailbreak it. The Dev Team didn't even have to do any work on the now famous RedSn0w as the 0.9.5b5-5 version can jailbreak iOS 4.1 beta.
Instructions to jailbreak your iPhone iOS 4 with RedSn0w remain the same. However, remember this is still a beta version. To date, this version of RedSn0w is only compatible with the iPhone 3G and iPod Touch 2G (non MC models).
Also remember that iOS 4.1 beta is called "beta" for a reason. It is released for developers and the general public should stay away from it.
Apple just pushed the iOS SDK 4.1 beta to developers. This move is widely expected to NOT fix the antenna issue but rather hide it behind cosmetic changes. This beta version (for developers only) will show bigger bars and will fix the formula that calculates how many bars are displayed, which Apple said to be "totally wrong".
Unless you're a developer, you really have no business playing with the iOS SDK 4.1. beta. Also keep in mind that installing this beta version or the future official iOS 4.1 may compromise your chances of jailbreaking or unlocking your iPhone.