There is definately a day/night system in the normal world. The twilight is always in the hazy-like state and doesn't change, but normal hyrule definately has night and day. I even remember seeing a picuture of Ordon village in the night.
EDIT: In Matt's preview he even says that "It's
twilight and a real-time sequence focuses on a blacksmith named Rusl and his apparent apprentice, Link". Thats taken straight from Matt and that is how
TP begins. So yes, there will be day/night cycles....and very advanced ones at that.