Ah, but the dead are not spread evenly over the planet. Most of them are going to be in the Middle East, east Asia, and (to a lesser extent) Europe. Since the US population grew so fast because of immigration, I bet we do outnumber the dead. Heck, if we start seeing zombie vikings, we'll even solve a mystery or two!

And some cultures have been burning their dead for centuries -- while their undead uprising will probably cause emphysema, it probably won't be immediately life-threatening.

So, I guess we cluster in areas with less history or more pyromania, and go from there?