According to iH8sn0w sn0wbreeze, the program through which you can make a custom firmware 4.0 jailbroken from Windows, will be available just a few days after the official release of iOS 4.0 on Monday. Through this custom ipsw you will be able to upgrade to iOS 4.0 without updating the baseband and you will be able to use either ultrasn0w or blacksn0w to do the decoding depending on the baseband you have.
Of course, this time frame is only an estimate and is based on the fact that iOS 4.0 final will have the same security holes as iOS 4.0 GM, but if the final version will have those "holes" closed then the appearance of a jailbreak solution would it could take a little longer than everyone anticipates.
I remind you once again not to update to iOS 4.0 as soon as it appears because you will update the baseband and you will not be able to decode it until another decoding software appears.
