I have a new Macbook Air which qualifies for the free OSX upgrade. I also have an older iMac running Snow Leopard. Can I upgrade the iMac without paying for the ML upgrade once I upgrade the MB Air?
Yes. Using the form when it comes back up will get you a redemption code. I'm assuming you use the same App Store account for both computers, of course. If you do, it'll update both no problem.