ASUS has made the boot loader unlocking utility for the Transformer Pad TF300 available from their website. The tool is an Android apk that you install like any other app, but when ran the boot loader of the device is unlocked -- allowing the installation of custom firmware. This comes at a price though, that being the potential loss of warranty. While I'm a firm believer that any warranty should be void if you want the ability to potentially ruin your hardware, there are many who see differently. But we all agree that you should have access to the hardware you've paid for, especially when it's not tied to a carrier's network. ASUS agrees as well, and that's why we love them.
Hit the source link, read and heed the warnings, and get to unlocking!