There is. There isn't a lot of point measuring arms as an indicator of body fat. But if you measure the right parts of the body, and you measure more than one, you can get a surprisingly accurate measurement of body fat - the US Navy Body Composition Assessment method gets to within 3% based on two or three circumferences and your height:

http://fitness.stackexchange.com/questions/1660/how-accurate...

There is, but I think if someone is using this app to keep track of their arm diameter, they are probably aware of that ;)