iPhone iOS 4.0.2 Downgrade Without SHSH Blobs: Not Yet Verified
The full method involves replacing the downloaded iOS 4.0.2 ipsw files with iOS 4.0.1. The process involves some searching and replacing of files. Therefore, involves risk of losing data on your iPhone.
The method is not yet verified by the Dev Team or any member like MuslceNerd or Planetbeing. If you are desperate to get your iPhone 4 or iPhone 3GS downgraded to iOS 4.0.1 and have no SHSH blobs saved then you can follow the instructions given here.
The better and safe way to downgrade iOS 4.0.2 to iOS 4.0.1 is given here, and you should first use this method before trying the one given above.