Update 12/18/12: If you’ve seen the film you’ll know that Thráin plays a part in the flashbacks at Erebor, and Moria, but not quite in the same way as in the text. I won’t go into details yet, but rather say that the scene shown below of Gandalf fighting Thráin is not in film one.
According to my web stats, I get more people searching for “Who is Gandalf fighting in the trailer?” than anything else! So, although this is only a hypothesis, the best guess is the scene shows a confrontation between the wizard and Thorin’s father, Thráin II, at Dol Guldur.
I’ve detailed my reasons for this here, but I wanted to take a moment to highlight the life of Thráin for those wondering who this grey ball of hair and fury may once have been (and if you think Thorin is a badass, wait till you read about his daddy!)