Is that you need to have ubiquitous Internet to make it work.
This looks like a neat device. But if none of my data is stored on the device, it is useless for reading email or working on documents. Wifi coverage is sporadic and expensive. Cellular coverage is even more expensive. WiMax (aka Clear) is only available in a few markets. Having to attach a dongle for Cellular/WiMax to the netbook destroys the form factor and fast start-up time.
Plus there is the DANGER of storing all your stuff in the cloud.