We have already seen iOS 4.0 Beta 4 (formerly iPhone OS 4) being successfully jailbroken and unlocked on iPhone 3GS and 3G using ultrasn0w and blacksn0w. But for this final release of iOS 4, both the tools are likely going to get an upgrade soon to make them full compatible. The iPhone Dev Team has already promised a full carrier unlocking solution sometime this month for 05.12.01 baseband. Geohot might also update blacksn0w unlock for this latest firmware, which has already been successfully ported to the Beta 4 of iOS 4.
While we don’t have the exact ETA on the release, what we know is that it will be arriving by the end of June, probably right after Apple has shipped first of iPhone 4 devices.