Wired has an interesting story on how dolphins say "hello." Apparently there's a "signature whistle" they use in social situations, when meeting up with dolphins from other pods. There also appear to be rules to the interaction, most of which are poorly understood, if at all.
I've been a science fiction fan for most of my life, so between books, movies, and television I've probably read or watched hundreds of scenes in which humans come into contact with aliens for the first time. Usually the process is completely seamless; as in "Close Encounters," there may be a bit of coordination at first, but soon the universal translator is working properly and communication proceeds apace.
This dolphin study reveals how hopelessly naive and optimistic that assumption is. We've lived alongside dolphins for tens of thousands of years, and we've been studying them pretty intensively for decades now. We've also had the remarkable opportunity to study entire generations of dolphins who lived in captivity. On top of that, dolphins are mammals, and so they share a certain degree of genetic heritage with human beings. With that wealth of information, insight, and reflection as backdrop, we've now been able to reach the conclusion that dolphins maybe, possibly say "hello." Otherwise, we know basically nothing.
If it's that hard to understand dolphins, with whom we have so much in common, how are we ever going to understand the attempts at communication by a species that we've just encountered, that evolved in completely different circumstances, and with which we have absolutely nothing in common? It's no wonder that science fiction makes this process so much easier -- those books would have been pretty boring if they had all featured long chapters of mutual incomprehension.