From First and Second Peter, Jude (Coming November 2011) by Daniel Keating, commenting on 1 Peter 2:6-8:
Peter shows that Jesus is the “stone” promised in the Scriptures, rejected by the leaders of Israel who put him to death (see Acts 4:10–11) but raised up to become the cornerstone of God’s house. The key issue is faith. For those who have faith, the stone established by the Father is “precious,” but to those without faith, this stone becomes a cause of stumbling and falling. What does it mean to stumble in this context? They stumble because they disobey the word, that is, they do not believe and obey the gospel (note the contrast with “obedience to the truth” in 1:22).