While there has been no official confirmation by Marvel, it is being heavily implied to the point where it's basically been confirmed. Aside from Mark Ruffalo joking around in an interview that he'd like to be in it...
...General Thaddeus "Thunderbolt" Ross (as played by William Hurt) will be joining the cast, according to the official casting news. As we all know, Ross is a major enemy of the Hulk and as such, there was a suggestion of the Hulk's presence in the film, but a recent tweet from Mark Ruffalo seemed to confirm this suggestion:
So, is Hulk coming to 'Civil War'? All bets seem to suggest 'yes'. The Russo Brothers do seem to think big and with Ant Man fighting in Giant Man form in the film being a rumour that is currently flying around, who else to fight back but the Hulk himself?
Sidenote: With the inclusion of Thunderbolt Ross, could my Venom theory (below) come to fruition and we see a Thunderbolts team forming after a certain someone gets a bit of a red tinge? (General Ross becoming Red Hulk for those who don't get it). Let me know in the comments.