Apple fixed the PDF exploit with iOS 4.3.4 which was used in the cable free jailbreak method : jailbreakme.com.
What you have to do? :

Head over to TinyUmbrella  immediately and save your SHSH files. If you haven’t heard of  TinyUmbrella before, all you have to do is download the corresponding software for your PC/Mac and plug your iPhone and ‘save’ your SHSH files, simple!  This is highly recommended for all iPhone 3GS, iPhone 4 owners.

(Or)

Launch Cydia on your iPhone and make sure you have your SHSH files saved over there [You will …

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 tightened security with iPhone 3GS and the latest 3GS hardware version is only capable of tethered jailbreak 🙁
Saurik, the man behind Cydia implemented a clever approach to bypass Apple’s insane security features of the 3GS! He explains the whole process in great detail here.
In short, all you have to do is :
Jailbreak your iPhone 3GS. Trust me it is easy, fun and super productive to jailbreak your iPhone and it is perfectly legal!
Launch Cydia on your iPhone 3GS
Click on ‘Make my life easier, thanks!’. If you don’t see it …