First of all, twospirits is talking about unlocking the bootloader and not just rooting, but:
Not all phones are the same mellimel, the kernel for the Photon was never released and nobody has been able to figure out how to get 4G WiMax working without the information contained in the kernel. Motorola wants their bootloaders to remain locked, so even if hackers/developers find ways to unlock the bootloader, they end up losing some functionality.
Your Nexus S had all the propriatary information released because having an easily unlocked bootloader is a common trait of Nexus phones, Samsung also leaves the bootloader relatively easy to unlock. HTC gives instructions on how to unlock their bootloaders. Motorola is just flat out stubborn on it. This could be why we have seen a HTC Nexus and 2 Samsung Nexi but nothing from Motorola besides the Xoom.
The thread at XDA regarding unlocking the bootloader is here: http://forum.xda-developers.com/showthread.php?t=1216143