A few weeks ago, I listened to a sermon by Mark Driscoll on 1 Peter 3:18-20, one of the most difficult to understand passages in the Bible:
Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit, in which he went and proclaimed to the spirits in prison, because they formerly did not obey, when God’s patience waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight persons, were brought safely through water.
A traditional understanding is that after he was crucified, Jesus descended into Hell. This is codified in the Apostle’s Creed, which I remember being exposed to in my two semesters at a Lutheran school:
Jesus Christ … was crucified, died, and was buried. He descended into hell. On the third day he rose again. He ascended into heaven and is seated at the right hand of God the Father Almighty.
John Piper has an interesting interpretation up that would make the narrative more consistent with the very clear passage of Jesus to the thief in Luke 23:43 (“Truly, I say to you, today you will be with me in Paradise.”):
With regard to 1 Peter 3:19, I take these words to mean that Christ, through the voice of Noah, went and preached to that generation, whose spirits are now “in prison,” that is, in hell. In other words, Peter does not say that Christ preached to them while they were in prison. He says he preached to them once, during the days of Noah, and now they are in prison.
I encourage you to read the whole post for a fuller picture of the evidence for this interpretation, as well as its consequences.