I had an iBook running tiger which is now mums. I brought a unibody pro running leopard in 2008 and have now upgraded to Snow Leopard. Would it be legal to use the leopard disc on the iBook considering the MBP is now running SL? I won't be installing it because the iBook isn't that powerful so will keep tiger but I was just wondering if it is allowed cause technically there is only one version being used at anytime.