DICE has now clarified the details surrounding the Physical Warfare pack, which was understood to be part of the pre-order bonus (and is). Apparently, you don’t have to pre-order to get the Physical Warfare pack because everyone will be able to download it, but it is apparently a timed exclusive.  This means that those who pre-order will get it right at release, but those who don’t will have to wait a bit before they can download it.

As for the “Back to Karkand” map pack, that is a bit different. If you want the Back to Karkand map pack, then you should pre-order in order to get it for free; otherwise, you will have to buy it if you want it. DICE claims that they have more than enough copies of the game to go around, so if you pre-order or buy it on release day, you should have no problem getting it.