Pretty sure I read somewhere that hair no longer disappears under helmets and hoods, so it's possible.
Look at the screenshot posted in this thread, it's pretty clear that it doesn't. Now, there's another thread debating that it doesn't look very good, but that's another subject, good or bad, it's definitely there, so yes, your hair is still visible under helmets in Skyrim, which was the main problem with having hats in previous games. You could maybe argue that you wouldn't be able to see your hair when wearing a helmet because it covers the hair, I'd argue this isn't always the case, but at least no hair in a helmet doesn't look as awkward as a hat without hair would.
This certainly doesn't prove hats are in, no screenshots have shown them, at the very least, but it does, at least, mean they'd be feasible, at the very least, it likely means that modding them in wouldn't be difficult.