If you are one of the lucky nearsighted, as Texrat mentions, it is just a matter of removing your glasses and holding the device close to your eyes, and you have a view like a car at a drive-in -- remember those? It can be awkward sometimes, like when you are trying to hold your tablet, your glasses, AND a cup of coffee, but there are ways to make it work.

If you are far-sighted, removing your glasses wouldn't work. To the extent that you could read a normal book, in a pinch, without glasses, I would expect you could read a tablet without them. However, it probably wouldn't be particularly fun. I would guess that most people could use the phone even blindfolded in many cases.