I saw this story on CNN's web page. Basically two swimmers off the New Zealand, found them selves being surrounded by large 'fish', which at first they though where Sharks, but in fact they where Dolphins. Nothing to unusual there Dolphins do seem to be interested in human swimmers. However when one of the swimmers 'broke out' of the circle he was herded back in by the largest of the Dolphins. Odd behavior for sure, but then it became apparent that there where indeed Sharks in the area and the Dolphins where protecting the swimmers from them.
This begs the question why?
The story mentions that Dolphins can show this sort of behavior when protecting there young from Sharks, but I'm not so sure. Dolphins are not stupid and I'm sure they can tell a human from one of their young. For a start think about a swimmer treading water, they would be upright, where as a Dolphin would be horizontal in the water. Also Dolphins sonar is through to be very 'high resolution' and since we are mostly water, they can see inside us. I'm not expert on Dolphin biology but I'm sure internally we are not that similar to Dolphins.
If they didn't do this because they though they protecting there young then why did they do it?
To the question I have no real answers. Maybe they are more intelligent than even we give them credit for (I thinking of a certain hitch-hikers guide here!) and they are just showing kinship. If this is the case then maybe they should do this more often and we might (but some how I doubt it) show the same kinship toward them.