According to the Dev Team:
Just like our other betas, this one intentionally doesn’t “hactivate” the iPhone. Please wait for Apple’s public release of 4.0 for that part of it.
YOU SHOULD STAY CLEAR OF THIS BETA SOFTWARE IF YOU RELY ON A CARRIER UNLOCK. That’s because the beta redsn0w works only if your device has already been upgraded to the stock 4.0beta1/beta4/GM IPSW from Apple, which contains a baseband update. If you installed that IPSW, you’ve already lost the carrier unlock until the next planned release of ultrasn0w and blacksn0w.
More information can be found here.