Skip an OSX version with App Store upgrades? I have a MacBook running Snow Leopard and newer MacBook Air running Lion.  If I buy the Mountain Lion upgrade through the App Store on my Air, will I be able to use the same App Store purchase on my older MacBook?

Or are OS upgrades not multi-machine friendly (somehow different from other apps that are pay-once download-many)?
You can upgrade to Mountain Lion from both Lion and Snow Leopard (SL). With SL you will need to be running the latest version 10.6.8 and then you can just buy it through the app store.

check this link for more details:
Apple OSX Upgrade

You just need to make sure your machine can handle Mountain Lion which you can see with this app from the app store Mac Tracker - its a free app.

OS' are like normal apps - they can be installed on up to 5 machines.

Good Luck on the upgrade:-)