Grey’s Anatomy Season 5 Episode 1 (S05Ep01) is to start on September 25, 2008 with a 2-hour season premiere titled "Dream a little dream of me". Here's a promotional video or sneak peak showing Dr. Derek Shepherd seems dead. Is this for real? Check it out and see for yourself.
In my opinion, I don't think the writers will kill one of the TV series' main character. I think it is only Meredith's dream or hallucination... just guessing, though ;0
Grey’s Anatomy Season 5 Preview Video