I went to a normal morning session at the cinema with my newborn DS last week and it was absolutely fine.
He just breastfed and then slept through the whole film while I cuddled him. He is a pretty laid back baby though who doesn't cry much.
There were also only about half a dozen people in the cinema with no one in our row as it was a weekday at 10am.
I would say it depends on your baby and the type of film, eg. an action film may be too loud in parts.