Using OS X of a new MacBook on an "old" iMac I have a late 2009 27" iMac. The iMac runs the default OS X version, 10.6.

This week I bought a new MacBook with OS X 10.8. Can I install the new OS X on my iMac without buying 10.8? Does the license allow this, and if it does, how do I do it? If you know how I can register OS X with my Apple ID, I can install OS X 10.8 via wifi.
The licence the install of OS X included with a new Mac is exclusive to that Mac, you can't re-use it on another. Buying Mountain Lion from the App Store entitles you to install it on all of your personal Macs.