Theories on the true intelligence of dolphins abound. One important starting point for these theories is the size and weight of the dolphin brain. It is roughly the same size and weight as a human brain. It is constructed differently and has fewer convolutions, but the underlying neurobiology at the cellular level suggests there is no reason why dolphins should not be as "smart" as people. It depends on the definition of "smart" and the recognition that "smartness" can evolve in different ways under different conditions.
Another reason to suggest that dolphins are highly intelligent is their speech.
Before de-classification, some of the most secret work done by the United States Navy (and the Soviet Navy) was geared towards utilizing dolphins as weapons of war. The work which trained dolphins to carry bombs on suicide missions against enemy ships has thankfully been stopped. Even so, during Operation Iraqi Freedom, it was well reported that dolphins were being used like bomb sniffing dogs to detect underwater mines. See the sunlink.com report here.
Finally, the affinity of dolphins for humans is well known and goes back at least to the times of ancient Greece when stories were written that told of dolphins saving drowning sailors at sea. Dolphins will instinctively do this when they see a human (or another dolphin) in distress, underwater. They push the injured one up to the surface so they can breathe. Also, in general, dolphins in the wild, or under any condition are not afraid of humans and will in fact, interact quite a bit.
This bond between dolphins and humans is fairly unique between animal species in the wild. For those who have actually seen dolphins or spent time learning about them or have been fortunate enough to interact with them, the sense of kinship felt between us and them is undeniable: Our brothers and sisters from the sea.