I was at my local gamestop when I saw this page that was very Pokemon in a book the employee's had.b They'd left it on the counter so I took a closer look. It listed a Celebi Giveaway happening in-store for all 4th gen games. There was some mention of a bonus for pre-ordering but it didn't say what.
The event is going to run from February 26th to March 6th. It will apparently unlock something in Black and White. It's also very likely to unlock an event in HeartGold and SoulSilver. There was only one Celebi event listed for this generation over at Serebii.