With SHIELD, NVIDIA made the decision to support the open source/root/Android modding community and embrace the hack-centric nature of the platform by making the device unlockable and easily modifiable. Now, it has made the necessary files available to really open it up for devs: the open source binary drivers and stock recovery image. Together, these files will not only allow developers to start tinkering with the device, but also flash everything back to its stock state should something go awry.
For those who are new to SHIELD development (which should be basically everyone, right?) NVIDIA does a good job of outlining how to use the files found in the download package. If you want to go a step further, you can also join NVIDIA's Tegra Registered Developer Program, which will get you early access to information and content, as well as open the doors to the SHIELD team to discuss issues and request new features. If you're a dev and a SHIELD enthusiast, it's definitely something to look into.
In the meantime, though, head here to grab the SHIELD goodies and start modding.