With the excellence of "Lost" and "Battlestar Galactica" of late, it's been easy to forget how long "Supernatural" has been off the air. Granted, it's only been several weeks, but this series always ranks highly with the rest of the character-driven shows, and the absence has been palpable. It feels a lot longer since the brothers were at each other's throats.
As one would expect from these writers, the events of the previous episode aren't forgotten. Dean certainly remembers Sam's rant about how Dean is holding him back, and Sam continues to delude himself regarding his good intentions for his demon-spawned abilities. Carrying that baggage into this particular battle in the longer war meant that every action had a deeper context.
On the face of it, the Brothers Winchester are still working together, but the reality is that the rift between them has grown. Dean doesn't like the fact that Sam is lying to his face, and Sam doesn't seem to have a problem with keeping Dean in the dark. On Dean's end of the equation, it's not particular surprising. Castiel told him that keeping Sam in line was his responsibility, or the angels would deal with him along with the rest of the threats.