As this story from MetroFanatic.com first pointed out, the Red Bulls have indeed wrapped up a playoff spot following this weekend's games. Basically, it would be impossible for both Chicago and the Galaxy to get past New York since they play each other on the final day of the season. FC Dallas clinched a spot as well following a 3-1 win over Columbus.
So the field looks like this:
1. Chivas USA
3. FC Dallas
1. D.C. United
2. New England
3. New York
Kansas City and Chicago are the two likeliest teams to get in but Columbus, Colorado and Los Angeles still have opportunities.
As far as positioning goes, I'd be surprised if these three teams changed positions by season's end so we're looking at Houston-FC Dallas and New England-New York matchups in the first round and Chivas USA-??? and D.C. United-??? as well. I don't like the ???s to advance, though Chicago would be a frightful first-round matchup.