Instead of trying to re-write / summarize the basics of an article on the subject, let me point you to an online article that goes through the different possibilities. Written by Dr. Stephen Stricker, it discusses the evaluation and treatment of children with tiptoe gait. He talks about habitual toe walking and also mild spastic diplegic cerebral palsy, as 2 quite common conditions. Then he goes on to discuss the less-common possibilities (such as autism, schizophrenia and finally spinal cord anomalies and juvenile type multiple sclerosis).
Go to: http://www.mastersofpediatrics.com/cme/cme2005/lecture30_1.asp