My son was also on Focalin and was crazy hungry ALL the time. I have to say over the years that with each of his different meds (straterra, focalin, concerta, ridilin la, etc.) he was always starving. Although I had asked the nuerologist and pediatrician about this at every appointment they always claim that it wasn't the meds. I have a neighbor who's son is exactly the same way.
My thought on it is that if you have ADHD and your taking meds to help you sit still in class then your not really going to want run around outside and play as much, which doesn't help anything either. But I know what you mean, my son could eat all day.
I took my son off of his meds this summer and it took about 3 or 4 weeks and it is amazing the difference in his appetite!! No one can tell me it wasn't his meds!