astah* UML pad for iPad has been getting plenty of positive feedback since its release on last winter, is free and supports Class diagram. We are surprised (and glad at the same time) to hear it when users tell us that they are actually willing to pay for it. You can see the Demo movie of astah* UML pad from here.

astah* UML pad supports not only the creation of Class Diagram with simple but intuitive UI and PNG export of the diagrams, but also the export of XML, and you can import the XML into our desktop edition, called astah* professional and continue working on the models you created on iPad on it. It is very easy to do so.

1) Send XML of selected diagram from the top right menu: [Send as XML via email]
















2) Import the XML into astah* professional from [Tool] Menu
(Yes, astah* works on Mac OS.)












So with this ability, you are able to do the initial modeling to put your ideas roughly on iPad and then continue working on it to fine it to complete on desktop.

If you would like to try, astah* UML pad is free and available on iTunes Store, and you are able to try out astah* professional for free for 50 days with an evaluation license key.
Get an evaluation license key

Cheers,
SJ