If you either have a new iPhone 4 with iOS 4.0.2 pre-installed by default or upgraded to the new firmware without saving your SHSH blobs first, here’s a trick that might allow you to downgrade to iOS 4.0.1 thus allowing you to jailbreak your iPhone via the JailbreakMe website. Emphasis on the word might as this hasn’t been confirmed to work as promised.
In a nutshell, you just have to slightly modify the official iOS 4.0.1 ipsw file, and then transfer all the contents to the iOS 4.0.2 ipsw file. This will allow you to fool your iPhone 4 and iTunes into believing that it’s installing iOS 4.0.2 while the truth is its iOS 4.0.1. Theoretically, it sounds plausible, but as we all know, plausible is not a guarantee that it’ll work.
For the full instructions on how to go about it, you can find it by going to Redmondpie’s site.