The current technique was finagled from existing jailbreak software (Pwnagetool, specifically) by msft.guy. On top of this prerelease hack, the iPhone Dev Team is claiming to have an unlock of their own nearly prepped for iOS 4 final, to be released within a month (presumably to coincide with the release of iPhone 4). In other words, come iOS 4′s official release, you should be able to jailbreak your handset—or at least your iPhone 3GS. (Nobody’s been able to test the exploit on an iPhone 4.
Instructions for trying the current jailbreak, for 3GS only, are available here, but it’s probably a bad choice: The final release of the software is set for the end of this month, at which point the jailbreaking process will likely be streamlined (and anointed by the Dev Team), and after which your jailbreak will last longer than a few weeks.

Quotes from the Dev Team:
“
Around an hour ago the new version of the iPhone Operating System (now called ‘iOS’) was released.
iOS 4 is a huge release for Apple with many many changes and those changes offer slick additional features.
These new features are being offered by Apple as a free upgrade to qualifying devices.
We are working hard on a release to our tools that will jailbreak your device (or give you iOS 4 via the jailbreak train) and provide you with a carrier unlock.
Until these tools are released you should hold off on updating your device until we have fully tested our tools with all the relevant devices.
If you rely on hacktivation or a carrier unlock (ultrasn0w) then you should not  upgrade until we have fully tested and released our tools.
Don’t be tempted with unofficial PwnageTool/redsn0w bundles or releases, just check here or our team twitterfor real time release information.
Remember, we risk our devices so you don’t have to (but this time anyone who has made a mistake upgrading their baseband firmware should be OK real soon ;)  Now!
For now, the redsn0w beta release supports only the iPhone3G and iPod Touch 2G at today’s 4.0.  It’s still a beta, so you’ll need to let Cydia reorganize, reload, and update after using redsn0w.

http://www.r4i-dsi-xl.co.uk

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