New addition for my list of UML tools for mobile devices : Draw UML for iPad assists you to create most common UML diagrams using your iPad for around 1$ (as seen today in the iTunes App Store). Important to note this is a drawing tool for UML and not a UML modeling tool .
Some additional screenshots can be found here
We can only expect to see more and more UML drawing tools for this kind of devices. I see them more as an aid to quickly review existing models or to sketch small diagrams as part of a team meeting but who knows?