I have a similar problem. I've got insurance on my current car until January 2008, but my ST should be delivered mid-March (dont have an exact date yet).

After speaking to my current insurers they advised me to cancel my policy, and they'll give me a refund for the remaining months of the policy minus an admin fee, then I'll simply setup a new one (although I'll be using a different insurer for my new policy with my ST as the price with More Than is too expensive for me!)