Heaps of interesting maps are being bandied about right now, and Tim Pierce has most of them gathered together at http://www.livejournal.com/users/topaz_munro/36843.html
My hypothetical favorite would still be a purple cartogram by county. One by congressional district would be a start, and there's a cartogram for that at http://www.ncgia.ucsb.edu/projects/Cartogram_Central/goal.html , but I don't have the necessary data to hand.
I think my favorite existing map is one tinlail pointed me to, which shows votes as color and voter density as lightness/darkness, based on the 2000 election. http://obsidianorder.blogspot.com/2004/10/red-and-blue-mapping-votes.html
I like it so much, it's below the cut. Click to enlarge.