I took the advice from the guys on DR1 and turned up at the Orange shop last week with an old unlocked tri-band phone and my passport. I got a SIM card for DR$350 which included RD$100 of calls (approx 8 minutes). I decided to purchase an additional RD$1000 because I talk alot. Anyway it all took about 30 minutes, fantastic service in English which helped and was a very smooth and pleasant experience. I had to wait until 9am Monday morning though as the shop is closed on Sundays.
I got all the information I needed from these guys on DR1 and will be eternally grateful because it made my trip so much easier ..... so thanx you guys.