I've been wondering the same thing, having seen conflicting info. The US Gov't CDC (http://www.cdc.gov/travel/caribean.htm) says you need to take malaria medication if visiting rural areas -- which would suggest that you don't really need it if you're only going to Punta Cana. However, Frommer's Caribbean 97 says you need to contact your doc 8 weeks in advance for malaria medication for the DR in general.
In any case, I'd recommend bringing along a strong mosquito repellent. I usually only use skin-so-soft or green ban, wishing to avoid the chemical DEET. But I fear I'll need to use that nasty chemical in the DR to be safe since we'll driving from Sosua to Las Galeras to Santo Domingo. Not sure if you need that strength for Punta Cana, though.