If you pre-order the Battlefield 3: Limited Edition on PC then EA will give you the Back to Karkand expansion for free.
Back to Karkand contains four "boldly re-imagined" Battlefield 2 maps as well as weapons and vehicles from the old game. There are also unique rewards and Achievements/Trophies tied to Back to Karkand.
You can pre-order the Battlefield 3: Limited Edition for Ł39.99 over on the EA Store.
The pre-order announcement coincides with the first gameplay footage of Battlefield 3. It's been a long time coming and you can watch it below.