I've actually been interested in this myself. In it's current state, ROS and Android are not as mature as ROS on Ubuntu Desktop (x86). There is an implementation of the ROS client library and a ROS master in the form of rosjava, but there are no ports of apps like rviz or the ROS nav stack yet for an Android environment. If you rooted your Android device, in theory you could crosscompile and run C++ nodes using roscpp. However it'll likely be a pain as all dependencies that we take for granted in the Ubuntu repositories (e.g. OGRE for rviz) have to be compiled manually. Your app will also not work on a stock Android device. You could also turn your ROS code into C++ libraries and invoke functions from a Java shim via the Android NDK without rooting the device, but that's another world of hurt.
I would recommend instead getting an x86 based tablet with an i3, i5, or i7 processor and intel hd graphics (e.g Acer Iconia W700). Ubuntu's Unity environment has pretty good multitouch support. The power draw is of course going to be more than an ARM-based tablet, but you'll get much higher performance. Be wary of Atom based tablets based on the Atom Z2760 processor. These use Intel's Cloverfield architecture which does not work with Linux due to the use of a proprietary PowerVR video chipset. The next generation of Atom based tablets should support Linux but that likely won't come out till end of this year.
Also, I'm not sure if Ubuntu Tablet would help. To my understanding, it's currently only a Developer's Preview. I'm not sure how much more different their environment is from desktop Ubuntu and if their repositories will be as comprehensive. In the future, it may well be the way to go for ROS development on ARM tablets, but I think it's way too early to consider it.