Just found this in the app store might be worth a punt App Store - Dominican Republic GPS Map Navigator
Don't hold your breath, Garmin's primary biz is hardware....i.e. as in your turn by turn GPS unit in you car, they thus keep their hardware and mapping proprietary. Apple is a direct competitor.I'm waiting to see if Garmin will write an app for iPhone / iPad. I have a Garmin device in my car and I use this map Dominican Republic GPS Map - GPSTravelMaps.com
It's not too bad.