My cousin was a toe walker. He grew out of it mostly by the time he was 3, but you could catch him walking like that until he was probably 5 or 6 years old. It's actually pretty common and most kids will grow out of it so don't worry too much.
Kids with cerebral palsy, muscular dystrophy or other nerve related diseases may walk on their toes. Children with autism might do it too. However, if your kid is developing normally in all other areas then toe walking isn't anything to be concerned about. Your kid will have awesome calf muscles though!
Audrey spent a couple weeks walking on her toes - followed by a day or two of only walking backwards.