When John Kerry lost in 2004, people left up their signs to signal that they were angry. They hadn't voted for Bush and Cheney. They wouldn't be responsible for whatever disaster happened next.
This time, people kept Obama their signs on their lawns as a symbol of their joy. They wanted to show the world that they had been a part of a historic moment. As in: Our guy finally won.
And now, as the cold, gray Michigan winter descends upon us and everyone braces for the shock of General Motors going bankrupt, we're keeping out our signs as a sort of a plea or prayer. Obama, don't forget us. We're out here in the dark. Please visit us, come down our chimneys, and bring us what we need.