The new generation of tablets are more about content consumption than creation. You can do limited creation, but I would not feel comfortable composing more than a tweet with them.
Typing on a surface that has no tactile feedback is both error prone and uncomfortable. There are some interesting technologies in the pipeline that might solve that. My feeling is that typing as a means of text entry is not the way to go, if you want to enter text like you're using a typewriter then you really need something that feels like a typewriter.
Tablet PCs and the predecessors of smart-phones allow stylus input. These allow you to either simply write your text longhand and for it to be recognised or, on the smaller devices, to enter text using a different notation. I've regularly used small handheld devices to compose long and complex e-mails, its slower than using a keyboard and comfortable to do on a plane, train or bus.
Today I'd say if you want to enter text you need a device with a keyboard, slide out keyboards are becoming increasingly good, or you need a stylus.
Tomorrow I think we'll start seeing some good innovation that will enable tablets to accept text as quickly or more quickly than a keyboard. That might be by some new technology or by revisiting some of the technologies from the 70s and 80s when keyboards were expensive.