The new message of the day on Black Ops 2 has just revealed that Ghosts will offer a total of 6 new unlocks (playercards and backgrounds) for fans who have played either BO2 or MW3.
Plus there will double XP for both MW3 and BO2 from Oct 25th 10am PT to Nov 1st.
UPDATE: @CallofDuty just tweeted out this image:
UPDATE 2: Activision has just released images of 4 of the 6 unlocks. These 4 are only unlocked if you’ve reached max prestige in MW3 (max prestige in MW3 is counted as 10th prestige according to ATVI) and/or Black Ops 2. There are two more that you get for just playing Black Ops 2 or MW3! (via DW247)
SOURCE: BO2 message of the day via @TWiiNSANE