Even though iOS 4.3.4 got jailbroken almost as soon as it was released, it came with a catch. The jailbreak was tethered, which means you cannot restart your device without having to jailbreak again. Unfortunately, it seems things are going to stay that way.
According to a popular iOS developer i0n1c (Stefen Esser, for those not fluent in leetspeak), Apple has not only patched the PDF vulnerability in iOS 4.3.4 but also added more mitigation checks. This prevents Redsn0w from performing an untethered jailbreak and it seems it will stay that way for a while.
So if you are one of those who upgraded your iOS in a hurry are in a bit of a disappointment. Others are advised not to upgrade since obviously the disadvantages far outweigh the advantages if you are a jailbreaker. You can only hope things will get sorted out once iOS 5 comes out.