IF you're going to see Pacific Rim tonight or this weekend, stay through the credits.
According to Lengendary's Twitter:
"When you go see #PacificRim this weekend, be sure to stay until the final credits roll. There's something waiting to surprise you..."
A Teaser trailer perhaps?
Update: I stayed at the end of the 10pm showing and nothing. Perhaps it was a ruse or it's not attached to every copy. Sorry to get your hopes up.