Hacker msftguy created unofficial PwnageTool which allows to upgrade to the pre-release of iOS 4.0 GM while keeping your jailbreak and old baseband.

This is not an official release by Devteam, so we recommend to wait for the final versions of both iOS 4.0 and PwnageTool.

For the advanced users who want to experiment here is a link.

Msftguy also provided a video:

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