At one time we were discussing about developing a simple simmulator for iPhone but then we decided that it is not worth it to maintain multiple code bases for different platforms. So we developed a web based simulator using GWT but with a form factor and bandwidth requirements that are more suitable for mobiles. You can check it out at http://webets.enthuware.com/
Looks like you guys had the same idea as me. I made a Java Class app too, which helps people preparing for their OCJP. I tried to work it out a bit. The app gives feedback on errors, keeps previous results and gives awards when certain numbers of answers are answered correctly.