Virginia, Missouri, Montana. Margins of victory around one percent in each of these Senate races.
This is when the flying monkeys in Diebold and Sequoia may be called. Just a tiny, tiny tweak in three precincts in those three states can give the Senate to the Republicans. One bag man with a laptop can move the totals in three aggregating PCs. And he doesn't even have to leave home to do it, though I think he'd be smart enough to use an unsecured wireless access point.
I'm betting on the cheat. It's too easy.