According to Apple, iOS 4.3.4 fixes the way the phone handles a type of font in the PDF file. Whenever such files are opened, malicious codes can sneak into your hardware. One of the main thing note about this iOS update is that it also prevents jailbreaking of your device using the web based JailBreakMe. It used this vulnerability to jailbreak your iOS device.
I'm sure that Apple wanted to fix their devices being jailbroken as soon as possible. That's the only reason I see behind this update.
You can update your phone by just plugging into your computer and running iTunes. Please note that you may not want this update if you've a jailbroken iDevice.
This update is available for iOS 3.0 through 4.3.3 for iPhone 3GS and iPhone 4 , iOS 3.1 through 4.3.3 for iPod touch (3rd generation) and later, iOS 3.2 through 4.3.3 for iPad. The update takes a while because it needs to download the entire iOS software than the patches only. Apple has fixed this in their next update, iOS5 and you can read more about it here.