The process in MA has changed by the way since Cinergi's post. You won't pay tax at all to Tesla now and will just make out a large personal check for tax to the RMV when submitting the RMV-1 form, certificate of origin and bill of sale to get plates. Tesla then requires you to send them a signed "Statement of Delivery Outside Califnornia" form indicating that the car wasn't delivered or sold within CA, excepting their need to take the tax.
I'll send them the form once I receive the car - they pre-filled the form as date of delivery 12/20/2012 and I'm still waiting for the car!