Interesting...the St Martin Tourist Office indicates you must have an IDP for driving on the French side since it is considered part of France. I suspect here in the US, insurance companies would react the same way if you were in a collision and cited for not having the IDP (or French drivers license). Here in the US you can pick up the IDP at AAA for $15 and another $8 for passport photos, and the IDP is good for one year. So if you don't have an IDP, the question is, "Do you feel lucky?"

St Martin Tourist Office - Driving Info