The Chicago Tribune calls it "a stunning upset Saturday that could be a sign of trouble for Republicans this fall." In The Chicago Sun-Times, the conclusion is that it's a "major blow to the Republican Party."
Or this from the New York Times:
The election here was expensive, bitter and, with its rash of television commercials, impossible to ignore. The national political parties clearly saw it as a critical one.By last week, the National Republican Congressional Committee had poured $1.2 million into this race; the Democratic Congressional Committee had given more than $620,000.
Bill Foster ran strong on Iraq the whole campaign, with several ads focused on withdrawal and the costs of the war. This was a heavily Republican district that turned on an anti-war and pro-civil liberties message to a Democrat.
Here is Foster's anti-Iraq war tv ad: