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.
And, I'm wrong on this. Wow, didn't expect
how wrong I was.
The new Airs don't come with a USB stick for recovery anymore. Instead you either use the recovery partition, or if your HDD gets entirely hosed, you can boot into an "Internet Restore" mode. That will launch the recovery mode over the network and let you reinstall the OS that way.
So it's a sure bet you won't be allowed to install Lion on the iMac just because you bought an Air.