yeah good point, it may be stored on an online server, but when you sign on the apps you are using are downloaded so you have a local version? if it was all online though I guess you would be a bit screwed.
it may not be a replacement for your laptop, not straight away anyway. it would probably start with specific applications, like business presentations, working while you're on the move etc. the idea will probably become more relevant when the use of iphones and blackberry type phones is more common and they become more powerful and probably also capable of faster data transfer speeds.
and I guess if you're using a laptop during a powercut you're on borrowed time until the power comes back on anyway haha my battery is shot, it only lasts 10 min without being pluged in now