There's a video of the Lion recovery here:http://www.youtube.com/watch?v=PPhoTUvdgac
It seems it's not a partition itself but it boots from a dmg on the normal partition. If your filesystem gets hosed, you won't be able to boot from the dmg but in this scenario, internet recovery takes over automatically.
It seems that you can erase your drive even with this setup - maybe it copies itself into RAM and then puts it back on the drive.
The good thing about the download idea for reinstalling is that they'll give you the latest version of Lion so you don't revert back to 10.7 every time. This can be good and bad though because an incremental update might break something and then you have no real way of downgrading.
It also means 2 hour reinstall times (depending on broadband) vs 20 minutes from an image on an external drive. Probably rare though and the USB drive coming in August isn't expensive.
I would like to have a USB pen too but when a new OS comes out, it makes the drive almost obsolete. It also doesn't help when you resell a machine and you've misplaced the drive. This move means that no matter what machine you've bought from Apple, you will always be able to reinstall the OS from now on.
I wonder if it will recover the latest one for free too. So if 10.8 comes out and your MBA drive fails, will it install 10.7 or 10.8 from the internet? Might be a sneaky way to avoid the upgrade cost.