Hmmm...
With Pervasive Developmental Disorder (PDD), a ballet class could go either way.
It's a very structured form of dance, with very clear expectations, rules, rote learning and repetition. It's almost ritualistic. Those elements might make it a good fit for a child on the spectrum.
Possible disadvantages might be that, if she has issues with motor contol, some of the movements may be difficult to achieve.
A possible long-term advantage might be enhancements in proprioceptive sense, motor control, balance and gait, all of which Autism Spectrum Disorders (ASD) kids typically struggle with.
Speaking from personal experience as a spectrumish little girl who was enthralled with dance, I was enrolled in ballet, tap and jazz classes when I was 4 years old. Hated ballet and quit after 2 weeks. LOVED jazz and especially tap (same rote learning and routine, same balance and stability, less difficulty due to hypermobility). I ended up sticking with dance until well into my 20s, and then picking it up again recently.
Your mileage may vary, but I certainly think it's worth a try. You might want to be open to other dance forms as well, just in case...
Trinity