its a good idea, but at the moment we wouldn't get very far as the average cell phone only has a 5MHz processor :D
java is a good idea though because most phones support it, even though it is a little slower due to the fact its run in a virtual machine
also daddygeek is right, it would have a massive impact on battery life, my k500 lasts a week if i leave it alone, less than a day if i use it all the time