Reality is what counts!!!
It is not important what everybody who posts here thinks, It doesn't matter what the law says or what posters try to interpretate into it.
What counts is reality, and that means if you get caught as a tourist, illegal resident and even as a legal resident driving with your foreign drivers license for more than 90 days you are in trouble. You can fix it with money (bribe) of course. But if you get involved in an accident and your paper work is not up to date e.g. you use a foreign license for more than 90 days, you better be ready to pay bick time $$$'s (US) or go to jail until you find the money.
Now, if you end up in an accident with a high ranking military officer, a government civil servant or just a Dominican with a big family behind..., well what could happen than goes beyond my imagination.
After more than 20 years living here in the Dominican, this is what I would recommend:
1. Dont overstay if you intend to drive!
2. If you plan to live here and need to drive, get your residency permit and then your drivers license ASAP!
3. Use the best insurance you can get and make sure it covers the "Casa de Conductor".
Happy driving everybody and watch out, it's Semana Santa.