One of my biggest pet peeves when talking about pre-order bonuses are ones that give you game content that you can't get in any way, shape or form if you didn't pre-order.  There are a lot of games coming out this fall (although that list has been cut significantly in the last couple months).  Its unlikely that gamers can pre-order everything.  I know I sure can't pre-order everything that I would like to.  Some games are just going to have to wait.  Giving away in-game bonuses that you can only get when you pre-order might seem cool if you're actually pre-ordering, but if you're not, you might feel a bit screwed over.

Its a good thing the retailer-"exclusive" pre-order content for Assassin's Creed 2 isn't going to stay exclusive for long.  If you pre-order the game you will receive two pieces of content that others won't get - Palazzo Medici and Santa Maria Dei Frari.  These are special areas within the game that you can explore and, likely, do plenty of things that assassins do, to your heart's content.

Producer Patrice Desilets recently talked to Kotaku about the "exclusivity" of these pieces of DLC and explained their plans for allowing everyone to have the content eventually.  "Eventually everyone will have the entire game."

So, there you have it.  Don't worry about pre-ordering the game unless you absolutely must show off your epeen to your friends by having the fancy extra areas as soon as you possibly can.