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 …