The Unpardonable Sin, what is it?

Sweet lovable Jesus says in Mark 3, "Whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin."

Helpful stuff:
1. Blasphemy against the HS (ie the eternal sin) is the ongoing and continual rejection of the witness of the Holy Spirit to Jesus Christ as Saviour and God. That is the eternal sin. It is constant resistance to the grace and mercy of God. --Geoff Thomas
2. here, by RIchard Philipps
3. Interesting post here, about some atheists who challenge folk to upload videos to youtube that supposedly (and wrongly!) condemn them to hell.
4. John Piper sermon on the unpardonable sin