Rob, ballet class is good for many purposes, but a cardio workout it is not.
Your heart rate has to be evenly sustained in its target range for at least 20 minutes for any activity to be considered a cardio workout. There are certain movements and combinations of movements in a ballet class that are certainly aerobic, but they come in short bursts. I wore a heart rate monitor to one of my classes to verify this.
If you start ballet, you will find out in the first 5 minutes that what you wear to class is only slightly connected with what you do. You will not have time to groove on your own dancewear or anyone else's because you will be far too busy assuming and practicing the correct postures and movements. [/i]