At this time there is still no official clarification as to the meaning of the line. However there are multiple places where people have suggested one of two things:
The first interpretation is that they all stayed in the Bellagio while conducting the raid, and there were 11 people staying in the hotel at the time, all raiding the mini bar, and since they are stealing from the hotel, they have no intention of paying the (from experience of hotel mini bar items) incredibly overpriced bill.
When Brad Pitt says, "Mini bar" he is referring to the fact that he is going to raid the mini bar and, (I think this is a safe assumption), not pay their hotel bill.
The other is that Rusty (Brad Pitt's Character) is constantly snacking throughout the movie, and it may be a real-world reference to the fact that they do not (did not at the time) actually have mini bars in the rooms.
For quite a long while after The Bellagio first opened in Las Vegas, they had "not yet installed their mini bars". I know because I stayed there and tried to order a mini fridge at least, but they claimed they had not yet reveived their mini bars, and would be installing them "soon".