I can't say what happens everywhere but I will share what I have observed here in suburban Detroit. 

I do a lot of driving around due to the fact that I have two young boys.  I see a lot of bumper stickers during my local taxiing. 

My observations are as follows:
I see Kerry/Edwards signs mostly on tiny beatup foreign cars.  I see Bush/Cheney signs on American vehicles (mostly SUVs and trucks) and the only hybrid I have seen in this area has a W in '04 magnet on it. 

I have never seen a Kerry/Edwards sign of any kind at the christian preschool my youngest attends.  I see no bumper stickers of any kind in my neighborhood, other than the occasional "My child is an honor student" sign. 

I abhor bumper stickers. But it is interesting social commentary on the kinds of cars that different kinds of supporters drive.