Apple just released iOS 4.1 update, not a super important update for 3GS users (unless you are into GameCenter!). iPhone 4 users can finally relax given that iOS 4.1 comes packaged with a fix for those nasty sensor issues.
Before you do any thing, please quit iTunes and do the following. TinyUmbrella is what you need to save your iPhone from Apple’s anti-jailbreak updates.
Download TinyUmbrella for your Mac/Windows/Linux.
Install/Launch TinyUmbrella and save SHSH blobs for iOS 4.0.2 for 3GS, iPhone 4 from Apple (or Cydia after the SHSH blob signing period by Apple is up).
Will make a detailed post about actually using the SHSH blobs for downgrading the iOS firmware.