It seems that a lot of people are confused about the bonuses you get for pre ordering, mainly the Nuketown 2025 (not zombies) map and the double xp that comes with it.
A unique code to download Nuketown 2025, which gives you double xp for the launch weekend too, comes with each LAUNCH COPY of Black Ops 2 i.e. the very first ones to be in the shop. If you pre order Black Ops 2 you will have a copy reserved from the first batch of games they get in on the 13th of November, a launch copy.
You could go into a shop and buy Black Ops 2 a few days later, without pre ordering, and you may still get the code for the download. Although in this case it's pot luck whether you get it or not.
Naming it a pre order bonus is just a marketing gimmick to ensure big sales right off the bat. Granted it guarantees you Nuketown 2025 but you could get lucky and get it without pre ordering. Hopefully this post makes it a bit clearer for some people
oh and here's a link to the official FAQ for any other questions you might have including the explanation about the whole launch copy thing.