*This post is rated PG-13.*
It's not really true.
There are a few instances of dolphins sort of sexually harassing humans (much like dogs occasionally do), but as far as a spreading scourge of murderous, raping dolphins--eh, not so much.
Dolphins are fiercely intelligent wild animals with incredibly complex behaviors and social structures. We can't pretend to be able to predict their behavior. For instance, not long ago, a trainer at Sea World was drowned when an orca (which is a type of dolphin) held her underwater.
To make absolutely sure that the dolphin rumor wasn't true and to separate fact from google search result, believe it or not, I emailed a dolphin researcher and asked her straight up if dolphins ever sexually assault people. Please take a moment and think about how embarrassing that was for me. Thanks.
Maddelena Bearzi co-authored one of my favorite books, Beautiful Minds: The Parralel Lives of Great Apes and Dolphins. The book discusses both groups of animals and describes how even though they live in very different environments, because of their comparable intelligence, they have some behavioral and social similarities. This book gave me several mindgasms. Needless to say, I recommend it.
There is also a great anecdote about a pod of dolphins leading Maddalena's research vessel toward a girl who was nearly drowning. I've seriously considered pretending to drown so that I might be saved by dolphins, but luckily, I'm too lazy to actually do it.
Since I so loved her book, I subjected this poor person to the most random email ever, but luckily she was nice (and patient) enough to answer my rather bizarre questions about dolphin sexual aggression rumors. She has my eternal gratitude and apologies.
She debunked the sexual assault rumor (as I explained above), but she acknowledge that a dolphin might curiously "play" with the occasional human because in dolphin world, sexual play is just a normal part of growing up and participating in dolphin society.
UPDATE 4/3/12: I kept my promise about her new book.
See info here: Dolphins Can't Read (yet), But You Can
She also stressed that we always need to remember that these are wild animals and need to be considered as such.