Speaking to the Dead
The Bible is clear that when you die you are "asleep".
And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.
- Daniel 12:2 (KJV)
So when you awake, you will either have eternal life (in Christ), or punishment in the lake of fire followed by the second death (from which there is no return, e.g. everlasting).
Given that the dead are asleep, it is simply impossible to talk to them. This is an important point.
Who are the mediums talking to then?
As it turns out, mediums are talking to demons.
For demons it's a great opportunity to introduce new doctrines, e.g. reincarnation, the "spirit world", we all go to heaven, no accountability, etc.
This is so dangerous (in many ways), that the Bible forbids talking to "the dead" (really demons pretending to be deceased loved ones).
There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch. Or a charmer, or a consulter with familiar spirits, or a wizard, or a necromancer. For all that do these things are an abomination unto the LORD: and because of these abominations the LORD thy God doth drive them out from before thee. Thou shalt be perfect with the LORD thy God.
- Deuteronomy 18:10-13 (KJV)
Regard not them that have familiar spirits, neither seek after wizards, to be defiled by them: I am the LORD your God.
- Leviticus 19:31 (KJV)
Amazing Facts has a few things to say about "speaking to the dead" as well. Check out this radio show excerpt.