If you wish to run normal OS X, you'd lose 4GB from the start. Then media will require a lot of space. Applications..
Then you may have to increase space, but that increases cost. If you instead slim down the OS, you'll get outcrys from people wanting the full thing - the same people who want full OS X on the iPhone.
The size of the machine makes it tempting. Worth noting that their current MacBook Pro and MacBook keyboards wont fit that size, those templates only go down to 12" machines e.g: PowerBook G4. Apple being a stickler for unity across designs will probably grumble at that.
Back To My Mac is the missing link which makes an inexpensive Apple subnotebook possible.
This single-handedly eliminates the need for large HDs in subnotebooks (or any notebook).
9 or 10" screen
near full-sized keyboard
16 GB SSD
The above would make for a great accessory computer. Net effect: All the utility of having all your files with you...without having your files physically with you. The ability to have all your files with you, but without having to pay the heft & cost premium of having a large HD or SSD.
If the Foleo will have the ability to access files off of a main computer and the ability to manipulate those files, let me know...