Astah UML Pad for the iPad has been getting lots of positive feedback since its release last winter; it is free and supports Class diagrams. We were very pleased when users said that they would pay for it- but we are still happy to give it away for free. You can see a Demo movie of Astah UML Pad here.
Again, Astah UML Pad supports the creation of Class Diagrams with a simple but intuitive UI and export via PNG or XML. You can also easily import the model into our desktop edition, Astah Professional and continue working with the models you created on your iPad.
How to import models from iPad to Astah Professional
By using Astah UML Pad and Astah Professional together, you are able to start your rough modeling on your iPad, wherever you are, and then refine and polish the models on your desktop.