The Raiders pissing away a 13 point lead against Detroit hurts. Detroit has to lose both, but they play SD (who is looking to have won tonight, so they still have an outside shot at the division and thus something to play for) and GB in GB on the final day, with the Packers probably favourites there and tuning up for the playoffs. That happens and the Seahawks get in.
(assuming SEA wins the next two)
There's another more wild scenario where the Lions win one of two, but the Bears win the last two, the Falcons lose two, and the Seahawks somehow get in on a tiebreaker.
But yeah, the Cardinals and Seahawks are 7-1 in past 4 weeks combined. The overhyped NFC east gets far too much airtime and media confusion on why those average teams lose games. There is a chance that the Eagles could still get a spot over Dallas and the Giants, which I would love actually.
This is a great link to see the playoff scenarios play out:
http://espn.go.com/nfl/playoffs/machine