What could a high, arched palate be a sign of other than marfan's syndrome?

Lots of things. We see high narrow palatal vaults in children with digit habits, sinus problems, enlarged t;a, and in those who inherit this shape from their parents. Seen in many syndromens. In children it's easy to fix. In adults the fix is a bit more complex. Ask your dentist to refer you to an orthodontic specialist who can evaluate how the skeletal and dental parts of your face come together and function.
Sometimes normal. Normal people sometimes have a high-arched palate. One should not worry about Marfan syndrome unless one has several of the physical appearance features (then a doctor's evaluation should be obtained). Having just one of the physical appearance features in an otherwise normal individual does not necessarily suggest Marfan syndrome or any other problem.
Environmental. Due to environmental factors affecting breathing in growth and development, these may contribute to palate shape in people who mouthbreathe. Just because there is a high palate, it alone is not a sign or a predictor of an underlying condition. If you are healthy, breathe well, and show no other predilection to marfan's or any other syndrome, then I would not be too concerned.