Asus releases official bootloader unlock tool for Transformer Prime
Asus had promised this tool a while ago and has come through. A couple of warnings come with the tool though. First, it should only be used with a Transformer Prime running Android 4.x Ice Cream Sandwich. So, if for some reason that is well beyond our understanding, you haven't yet updated your device to ICS, don't use this tool. Also, Asus makes it very clear in the warnings that it doesn't approve of bootloader unlocking and that there are "risks" which you would be taking on, not to mention that your warranty would be void and you would no longer be able to get support for Asus if you unlock your device.
All that said, if you want the tool, you can find it on the Transformer Prime support page under "Utilities".