Some owners have built custom locking systems, while Apple uses an ethernet-based security cable at its Apple Retail Stores that doesn't actually lock the device, but it does set off an alarm if someone tries to steal it.
For those uninterested in the "do-it-yourself" look, MacLocks.com has created a new security bracket that tucks inside the edges of the rear access panel, around the power cord, adding a more permanent security solution to the Mac Pro.
The new Mac Pro Lock Bracket with Security Cable Lock fits comfortably underneath the cylinder lid to offer complete protection of the device and its hardware components. By designing the Mac Pro Lock to fit the curvature of the new Mac Pro and the bracket plate to sit securely over the power cable the lid is now doubly locked into place. The bracket plate has an extended lip that will allow you to attach the included Security Cable Lock ensuring your Mac Pro workstation and all its impressive (and expensive) hardware are not going anywhere.The Mac Pro Lock Security Bracket is available for preorder from MacLocks.com for $90. The company estimates it will ship by late February.