Philadelphia announced that McNabb was on the trade block around a week ago, there was some speculation of which team was going to make a move for him, the Oakland Raiders were the front runners to trade for him but McNabb didn't like the idea of playing in Oakland, that open again the talks with other teams and the Eagles found a trade partner in the Washington Redskins.
The trade leaves Philadelphia with two QBs, Kevin Kolb and Michael Vick, the first one being the favorite to become the starter. Kolb has already step up when McNabb was injured and has showed a lot of promise, becoming the starter will let him groom with the other young talents in the team (DeSean Jackson, Jeremy Maclin, LeSean McCoy) in a move that certainly about the future of the team.
Also, Philadelphia now have 2 picks within the first 40 picks of the draft that adds a lot of value in young talent to address some of their needs.
Washington under new HC Mike Shanahan and OC Kyle Shanahan needed a reliable quarterback to help their project. Shanahan's offense doesn't needs elite talent at the offensive line to be successful but it certainly needs a good quarterback and they got more than that, with McNabb they get a natural leader for the team.
Now with McNabb on board the Redskins will look to address their position needs at the draft, that leads me to the next point:
Washington was the "favorite" to draft Jimmy Clausen and now that they got their quarterback is likely that they will look to draft a left tackle (or trade down). If Russell Okung is still available he will be the pick, but if he's not any other tackle could be picked there: Bryan Bulaga, Trent Williams, Bruce Campbell... they question will be who the Redskins like the most.
Jimmy Clausen could be picked quickly by the Seattle Seahawks, Buffalo Bills or Jacksonville Jaguars or he could suffer a free fall, à la Brady Quinn in 2007, and be picked 30th by the Minnesota Vikings (if Favre hasn't make up his mind by then).