iOS 4.3 is currently only available to developers or people with iOS developer account. Geo Hot on the other hand provided some pretty simple steps for general consumers to be able to update their Apple devices prematurely to the latest version which is 4.3
Disclaimer: I wasn't able to test this steps since I don't have device and I cant assure you its stability and quality once installed on your device. It is advised that before you continue with this process, you know what you are doing and you understand the consequences of your action. Proceed at your own risk.
Steps can be found at Geo Hot