How would you reinstall Lion if it's ever uninstalled on a new Macbook Air if this is the case?
Once Lion is installed it'll make a recovery partition that will assist in the reinstallation of Lion if needed. If the whole drive is replaced, the end user must first install Snow Leopard then update to Lion. It is currently unknown how an end user will reinstall Lion on a Mac the ships with Lion if they replace their hard drive.
All machines come with restore discs (or USB sticks in the case of the MBA) with the OS. I don't see Lion changing this. Although you might see more and more machines shipping with USB sticks instead of DVDs. And the restore is usually locked to the exact model it ships with.
Do the drop in discs supplied in the box now with all new machines (manufactured pre-Lion release) are machine specific?