I was thinking
what if the ultrathin rumours are more like a remote display - with limited onboard storage that works with your desktop. On the go it uses a simpler interface like the ipod at home it allows you to use a vnc type client, optimized to have function to your main machine.
The price point could be low, between the ipod touch and the lowend macbook...
in the air could be the connection to your main machine.
Just a thought
I love that idea, just like apple tv can access stuff on your main machine, why not take the concept a stage further. You could even have access to other macs or pcs, given you have password etc. Whilst we need more storage to hold our digital lives, why not detach it from the portable and sit it in your home/office!