Can I install the preinstalled Mac OS X Mountain Lion on other Macs?
No. The preinstalled copy of Mountain Lion is not licensed to be installed on additional machines.

From the Software License Agreement (2.A.):


  Preinstalled and Single-Copy Apple Software License. Subject to the terms and conditions of this License, unless you obtained the Apple Software from the Mac App Store or under a volume license, maintenance or other written agreement from Apple, you are granted a limited, non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer at any one time. For example, these single-copy license terms apply to you if you obtained the Apple Software 
  ￼￼￼preinstalled on Apple-branded hardware.


(emphasis added)

You will have to buy an upgrade in the App Store for your other Macs, however for non-commercial use you can install a single App Store purchase on multiple machines, given all the computers are in your control.