I personally believe that a good number of people who once lived on this earth have returned in one form or another to deliver messages or to do whatever else God commanded them to do.
I agree with Shiann, I’m sceptical of those who seem to speak with the dead out of a primarily economic motivation. I also believe that, although they love us dearly, by far most of the dead are somewhat busy doing the Lord’s work and aren’t just floating around waiting for somebody to call them up. I believe that C. S. Lewis put the situation well in his book The Great Apostasy (referring to the first, original Apostasy, where Satan and his followers either left or were cast out of Heaven). A rather controlling woman comes up to Heaven from Hell and demands that her son come see her. Her demand is refused as her son is unforunately busy on other things. But she is told that she can eventually go to him, if she’d like.
So, I tend to rather sceptically view spiritual medium who claim to be able to call up whatever dead person you’d wish to speak with (especially if this is how they make their living). I believe that Satan could masquerade as whatever dead person he wants. He did, after all, watch that person throughout their life. I think most mediums are merely somewhat deluded, though, not the target of demonic influences.