Sometimes the undo shake works, but most times it does not. Perhaps an "undo" button would be better.

This problem exists on both iPhone and iPad.

Thanks - spazzdog

I agree, it's not very consistent, and shaking my iPad is quite... I feel weird doing it in front of 50 other students in the bus xD
But I also would like to say something about it. Playing in buses, and the drivers not being very gentle with their machines, I have to undo quite often. I would enjoy being able to undo words, and to have two or three undo's instead of one.

We agree with you, shaking your iPad is not very cool looking.

We have added a button in the 1.21 update that just shipped to Apple.

For iPad only. Should be out mid-next week hopefully.