They people over at LEGO have updated the MINDdroid app that allows you to control your NXT robot from any Android device.
A few of the new features:
Upload a program to your NXT from your phone, no PC needed!
Control sounds and play music
Get sensor readings converted through text to speech
"What about doing xyz?" I hear you ask? We no problem at all, they have also released all the code as Open Source, so if you're good with programming, you can dive in and modify to your hearts content!
Open Source Software: https://github.com/NXT/LEGO-MINDSTORMS-MINDdroid