... that is actually not cracky in the least.
So, episode 6.11. Appointment in Samarra. For those who haven't heard, Dean makes a deal with Death: Dean will play Death for a day, and Sam will get his soul back. While Dean is dealing with Death, Sam tracks down Balthazar and gets the angel to agree to work a spell that will keep Sam's soul forever outside of his body. It requires the blood of his father, but John is dead, so Bobby will have to suffice.
Theory: Sam will drain Bobby and Dean will have to be the one to kill him. Yay for angst.