Currently Being ModeratedJan 13, 2013 12:36 AM (in response to CB1628)
You can download Mountain Lion from your App store account for any machine that you own and which is compatible with it, without having to pay again.
If you have it on the iMac already, then Just log into the App store from the MacBook Air, go to your 'Purchases' tab and hit 'Install'.
For a list of compatible machines, see
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