If I had a 10 inch screen, CPU, mem, storage and a network connection, why would I not want to be able to do development on it. I see irb has already been packaged for android, which makes it easier. While the iPad has some fancy restrictions around file systems and such, Android doesn't seem that far off from a normal computing environment. I don't see any churn or discussion about getting this stuff on tablets, so I thought I'd speak up.
I often have a need to make changes or write some code for someone when I am not at my desk. I also love to practice and going through things like Ruby Koans, Code Katas, etc. I currently carry my personal laptop (macbook) to work, because my work network blocks me from changing anything external to the company. But they do have a visitor network for personal devices. I've held off from getting a tablet device because neither provide any great option for developers.
I suggest Ruby, because it's still gaining ground and many Java developers, like myself, are shifting that direction. It shouldn't need as much as Groovy, Scala, etc. to get set up and would take up less space and CPU on a mobile device. Don't make me write an app myself, because it wouldn't be as nice.