Ok guys let me clear this upfront. I haven't tested this method yet, but by the looks of it, there is no reason why it shouldn’t work. If you have just got a new iPhone 4 or iPhone 3GS (new bootrom) which is on iOS 4.0.2, you can now downgrade to iOS 4.0.1 without having your SHSH blobs saved. This will of course allow you to jailbreak and unlock your new iPhone with JailbreakMe, and patch it with PDF Patch so that you are safe from PDF vulnerability in iOS 4.0.1.
Many of you asked for it and so here is the detailed guide on how to downgrade your iPhone 4, 3GS, 3G and iPod touch 2G / 3G running iOS 4.0.2 to iOS 4.0.1 / 4.0 / 3.1.3 / 3.1.2. The following step by step guide will help you rollback / downgrade to iOS 4.0.1 / iOS 4, iPhone 3.1.3 and 3.1.2 from iOS 4.0.2 as long as you have SHSH blobs saved for the respective firmwares for iPhone 4, iPhone 3GS, iPhone 3G and iPod touches.