1) Must have an internet connection
2) Keyboards destroyable by pupils
3) Netbooks costs about the same and run a full Windows OS
For me, there is something about the form factor of a tablet that, once you get to a 1:1 ratio, I don't think can be beat. A bit like every teacher having access to powerpoint, a projector and an IWB in every classroom.
That said, I want to like them...