Chip, I agree...and I usually wear long pants...even during the day...if Im doing any kind of business, but when Im just kickin it in Cabarete, Sosua, Playa Dorada, etc...I wear shorts if it's hot. This isn't Colombia or Brazil where you have to worry about being kidnapped if identified as a tourist...
There's no reason the average tourist here should worry about what they're wearing if they're strolling around a tourist or beach area. If they're gonna get jacked for a few extra pesos by trinket sellers or taxi drivers, it's gonna happen no matter the length of their pants, and most people I know who are on vacation aren't worried anout that. If shorts are more comfortable to you when it's hot, wear shorts... there's no reason to try to pretend you're a local...just....dont....wear...black...socks... with sandals...and save the speedos for sunning on your private balcony!
In Cabarete, most ex-pats I know, whether theyre surfers or millionaires with mansions in Sea Horse Ranch wear shorts day and night...
It is, however a different story, as mentioned above in Santiago and Santa Domingo, particularly in the evening and at the Historical and Religious sites...