The newly published ESRB ratings certificate for Battlefield: Bad Company 2 mentions that "players conduct missions on an unnamed Japanese island during World War II and then move into the modern day".
Earlier, we mentioned that Battlefield: Bad Company 2 would support weapons from last year's Battlefield: 1943 (pictured above). It sounds like this could be a beginning mission establishing the BFBC2 continuity, not a flashback level. But it could be that DICE is testing community reaction to the World War II genre, which of course many have complained is played out within FPSes. That said EA's ramping up a competitor to Modern Warfare, and this might tip off ideas of going after World at War.
No surprises in the rest of the certificate. "Characters use profanity (e.g., "f**k," "sh*t," and "a*shole") during cutscenes and in the heat of battle." As I said, no surprises.
Battlefield: Bad Company 2 [ESRB via Connected Consoles]